Don't really know, if some sounds work and others don't it could be anything. If all normal Minecraft sounds work all the time, but some modded sounds only work some times, then it's the mod's fault. Mods on 1.15 aren't fully developed yet, so it could be wait for an update time.
If all sounds work sometimes but then all sounds cut out entirely that could be anything from overloaded CPU to bad sound card to a janky connection between the sound card and the speakers. Hard to tell really.