I assume you've swapped both Pokeys? All of the sounds are generated by the Pokeys, so they are the heart of the audio generation.
Also, make sure you have all of the correct components installed around the audio preamps next to the Pokeys (where the LM324's are.) This is the section that must be modified when you convert from BW to Gravitar and vice versa. But a missing or broken part there could cause some lost sounds. Here's a summary of the mods, so you can know if any missing parts are supposed to be missing or not:
Audio mod summary for converting Gravitar to BW:
- Change C27 from 1000pF to 100pF (increases Low Pass Filter bandwidth, to play higher sounds)
- Remove C28 (1000pF) (disables LPF, to play higher sounds)
- Remove C34 (0.22uF) (disables LPF, to play higher sounds)
- Change R46 from 10k to 22k (changes mixing of the two pokey audio outputs)
The static suggests the Pokeys aren't being controlled properly, assuming the Pokeys are both good. (Test them in another game if you can, and don't throw any away until you are sure they are bad.) I'd check the control traces to the Pokeys, making sure you don't have a broken trace. See the schematics.
View attachment 855213