That'd be pretty cool, especially for Picture Window games.
/splay -b <filename>
The -b switch would blend a sound instead of stopping any currently playing sound(s), so they overlap.
Queued (-q) sounds would still wait in queue until any currently playing sound(s) stop playing. If you use -b and -q together, you would be able to queue any blendable sounds after a non-blended sound that's already in queue, so that they both play together.
/splay -b sound1b.wav
/splay -q sound2a.wav
/splay -qb sound2b.wav
/splay -q sound3.wav
Could also add another layer for background music that plays behind anything else that's playing or queued, with a -B flag. And -l to loop it.
/splay -Bl bgmusic.mid
Another approach would be to add an optional name to a playing sound layer, like timers.
/splay bgmusic -Bl greenhillzone.mid
/splay sfx sound1a.wav
/splay sfx -b sound1b.wav
/splay bgmusic stop
/splay bgmusic -Bl levelselect.mid
The default layer would be named "default".