Bakster
I am not sure I'd have the space under the stand. It is very limited.
They make speakers that fit in the "leftover" spaces in HO locomotives. Saw a high-fidelity speaker tha measured 5x15x2.5mm (wxlxd)
Sound cards appear to be similar. Check under DCC supplies and videos.
Also, you seem to need mp3 sound files for best results.