[FluidSynth] Music volume setting has no effect
When FluidSynth was playing music on its own (not via Doomsday's SFX backend), the music volume level was not being set at all.
This scenario occurs for example when FMOD is not available.
Fixed|FluidSynth|Audio: Excessive lag on music volume setting
Apply changes to music volume immediately if possible.
One more detail to note: because FluidSynth uses a buffer to store generated samples, changes in the gain setting become effective only after a short (second or two) delay. This may make the music setting seem unresponsive at first. A better approach would be to only resort to changing the gain if the music isn't being streamed manually to the SFX backend (in which case one can apply immediate changes to the channel volume).