Mod version 1.12.2 - 2.2.19.0, within modpack "Antimatter Chemistry" version 1.2.34.
Randomly when using the jetpack it stops making flying sounds.
Only relaunching the modpack will re-enable the sounds, even reloading resources with F3+T they do not come back

I've noticed this a lot when flying around in the Nether (which has a lot of ambient lava sounds and mob sounds). My guess as to what is going on is the game can only play so many sounds at once so it cuts certain ones out, and the mod doesn't know how to handle that.