@dankcushions: I noticed that hack in the RGUI menu as well. It was enabled by default for me.
@zerojay: This worked perfectly. Crackle is pretty much gone when volume is set to around 80% in the service menu. Now if only there was a way to get these volume settings to save so they don’t have to be adjusted each time…
Edit: So I’ve come up with a temporary solution to fix the low audio. I’ve added the line audio_volume = “25.000000” to the retroarch config for MK2 and MK3. This raises it to a level which is about equal to the default of most other games on my system.
Games are now fully playable. If/when support is added to libretro-mame to save nvram settings then the audio can just be adjusted there, but this works in the meantime and avoids having to adjust the audio manually each time.