Sound issue: it's a MorphOS bug, apparently related to its WBRun. Workaround: don't run the game via "launcher" icon (which is nothing but a script that ultimately does a WBRun on one of the real executables) but use the respective _MOS icon directly.
Flickering: again, apparently a MorphOS bug on your system. The game uses TinyGL for rendering on MOS. This includes screen and double/triplebuffer setups. So if anything flickers, then the culprit is there inside TGL.
Also, check out readme and relnotes, if you have further questions.