After starting music with /music, changing the game's Music volume will set the custom music's volume to that value, even though it began playing back with the volume parameter as a relative volume to the game's Music volume.

Steps to reproduce

• Join a world
• Go into Settings > Audio > Set "Music" to 50%
• /music play record.stal 0.1 0 play_once , this plays record.stal at 10% relative volume, now the music will play at 10%*50%=5% actual volume
• Go into Settings > Audio > Set "Music" to anything slightly different than 50%
• Now the music will play at 50% actual volume, ignoring the volume supplied in /music