I followed the directions that Mitsu posted about concatenating the sound roms to form vroma0, but the example he showed only had adpcma roms so it wasn't clear how to handle the adpcmb roms. I inspected the existing rom and noticed that there was 1Mb of zero-padded data between the concatenated "a" roms and the "b" rom. I started to inspect some of the other games that had "a" and "b" sound roms, and noticed that sometimes the filesize was bigger than the total of straight-concatenation, so there must be some padding going on. Then I noticed Cyber Lip looked like it was straight concatenation of a+b, so I tried that with Ghost Pilots. The sounds were off, but the melody was more complete. It seemed like it was accessing all of the sound data, but the wrong data. I recognized at that point that it was likely that the sound driver was accessing the zero-pad region for part of the music where we noticed the missing instruments in the original rom. So I tried it with 512 zero padding before the "b" rom and it worked!
The structure of the old (non-working) vroma0 stacked top to bottom:
v11 (1Mb, pcma)
v12 (512b, pcma)
zero pad (1M)
v21 (512b, pcmb)
---------------------
total size 3Mb
The structure of the new (working) vroma0:
v11 (1Mb, pcma)
v12 (512b, pcma)
zero pad (512b)
v21 (512b, pcmb)
---------------------
total size 2.5Mb