This section by Andreas Koch.
(POPS info updated 8/15/06 by mdc thanks to Lee Brilliant)

There is already a lot of stereo software for the upgraded Atari computers
available, of course most of these programs are limited to certain/special
upgrades and merely perform their stereo effects on these items (with
otherwise upgraded or non-upgraded Ataris, the sounds or programs will
only play in mono):

a) software for the various stereo-upgrades:

- 3 channels with one Pokey (POPS-software): As far as I know for this kind
  of upgrade, there merely exists a patched version of the Pokey player
  program, I am not sure if there is anything else for it; anyway, refer to
  ANALOG #66, November 1988, pages 54-60;

- stereo with two computers (thus two Pokeys): As far as I know for this
  simple trick there merely exist two programs, they are "Perestroyka" and
  "Sky Network" by T.Liebich. In order to achieve the stereo effect, you
  have to boot/load one of these demos on two computers (connected to
  different TV's or monitors, there is no need to connect the computers to 
  each other!). When done, press 1-5 on the first computer while pressing
  Shift-1-5 on the second computer. Meaning, if you want to hear the first 
  sound in stereo then press 1 on computer 1 and press Shift-1 on 
  computer 2 simultaneously (that`s a little tricky, I know). If you want
  to hear sound 5 in stereo, then press 5 on computer 1 and Shift-5 on
  computer 2 simultaneously. Tricky at first, but sooner or later you will
  get the hang of it. Of course you can also connect the two Ataris to
  a hifi-system, using the sound output of one Atari for the left channel
  and the sound output of the other Atari for the right channel...

- stereo-sound with Stereo-Blaster Pro (Portronic/AMC): As far as I know
  there was at least one demo disk (early version was single-sided only,
  later versions were double-sided), that contained some demo-software,
  namely the simple "Stereo-demos" (by AMC, side 1) and the
  "Stereoblaster-Demo" (by HU-Soft, side 2 if available). The Stereoblaster
  demo was written in Turbo-Basic and played back via Compiled-Turbo-Basic,
  it uses Chaos Music-Composer Sounds (*.CMC) and a few of these provide
  stereo effects, if equipped with a stereo-blaster-pro and a hifi-system.
  The simple stereo-demos included some programs written in Atari Basic,
  for example a (pong-like) bouncing ball and a flying helicopter. Equipped
  with a stereo-blaster-pro and a hifi-system, one could see the ball
  bouncing left and right and simultaneously hear the sound fx on the left
  or right channel. The helicopter started at the left side and produced a
  loud sound on the left channel, when it was flying to the right side, the 
  sound faded on the left channel and got louder on the right channel, 
  until the helicopter disappeared (and the sound completely faded away). 
  There were some more of these simple demos available, but I don`t 
  remember them anymore.

- Stereo with two Pokeys: There already exist dozens of sounds and demos,
  that support this upgrade, most of these programs were made in Poland,
  but a few sound-demos were also made in other countries. Anyway, the
  following programs support stereo via two Pokey chips:

  - Alf-Demo by the Unknown Base (Netherlands);
  - Alpha-Demo by GMG (Slovakia);
  - AMS-Stereo player by ??? (author unknown), USA;
    (there are at least two AMS-stereo-players, that let you play
     *.AMS sounds in true stereo or at least simulated stereo!);
  - Ballada sound by DJ V / BK (Poland);
  - Base 33 by AIDS (msx by Greg, Poland);
  - Chaos Music Composer version x.x patched by ??? , Poland;
    (=> the original version by Janusz Pelc / LK Avalon is only mono,
     but there is a stereo-patch available, as well as various patched 
     CMC stereo-versions on the internet);
  - (many) *.CMC sounds created by one of the many stereo-versions of 
    Chaos Music Composer;
  - Cogito-Demo by AIDS (Poland) 
  - Do you see the light? sound-demo by Roemer of UNO (Germany);  
  - Draconus, patched version by ANG and/or Micro Discount (NL/UK)
    (the original version by Zeppelin games is only mono!);
  - Dynakillers (Game) by GMG, Slovakia;
  - First of All (sound) by Raster, Czech Republic;
  - Impossible but Real Demo by MacGyver (Poland);
  - King of Aggregat by X-Ray / Slight (Poland); 
  - Megaplayer Versions 1.6 and 2.0 by MacGyver (Poland)
    (=> and thus all *.CMC, *.MPT, *.TMC, etc. sounds played with
     this sound-player tool can be heard in true or simulated stereo!);
  - Multi-Pro-Tracker 2.4s by Jaskier/Taquart, (original mono version
    by SoTE; thus *.MPT sounds can be generated in stereo!), Poland;
  - (many) *.MPT sounds created by the stereo-version of Multi Protracker;
  - Nazebany by DJ V / BK (Poland); 
  - Overload sound by X-Ray / Slight (Poland);
  - Raster Music Tracker 1.x by Raster, Radek Sterba (a PC program 
    that creates mono or stereo *.RMT sounds that can be played back 
    on the A8 or any Atari 800/XL/XE emulator);
  - *.RMT stereo-sounds created by Raster Music Tracker;
  - Stereo-Patch for Pokey Player by Chuck Steinman
    (=> thus all Pokey-Player / *.V sounds can be heard in stereo!);
  - Stereo-Patch for Softsynth by Freddy Offenga (Netherlands)
    (=> thus Softsynth will create stereo-sounds!);
  - Stereo Patch for World of Wonders by Freddy Offenga (Netherlands)
    (World of Wonders is a great Softsynth sound-demo!);
  - Still Alive (TMC-sound) by Greg, Poland;
  - Time sound by X-Ray / Slight (Poland);
  - Theta-Music-Composer version 1.x by Jaskier/Taquart
    (=> thus *.TMC sounds can be generated in stereo!);
  - Theta-Music-Composer version 2.x by Jaskier/Taquart
    (supports 1, 2 or even 4 Pokey sound-chips !)
  - (most) *.TMC sounds created by Theta Music Composer;
  - Vanity sound by Kuchara / Excellent (Poland) ;
  - Worms (320k-Demo) by Datri, Czech Republic;
  - Zybex, patched version by ANG and/or Micro Discount (NL/UK)
    (the original version by Zeppelin games is only mono!);
  - that's all what I found so far...

b) software for other sound enhancements:

- enhanced-sound with Covox: As far as I know this upgrade will playback
  digitized or sampled sound in 8Bit resolution rather than in 4Bit
  resolution. The following programs support the Covox-Upgrade:
  - Inertia 2.x, a MOD-player by MadTeam;
  - Inertia 3.x, a MOD-player by MadTeam
  - Inertia 4.5, a MOD player by MadTeam;
  - Protracker 1.5, a MOD-editor and player by MadTeam;
  - NeoTracker 1.x, a MOD+NEO+SMP player by EPI/Allegresse;
  - that's all I have found so far; 
note that all these programs will still work with pokey...