Maybe they got bad speakers. At least on real hardware with good speakers the sfx sound fantastic.
A shmup without sfx is a very bad idea. There are several C64 games where you can choose sfx or music. To play without sfx is to me hopeless in this type of games.
As you know C64 got 3-channels. It is amazing what people can get out of them but in this game often 2-channels are used for sfx.
It is a designers choice. To me this game is very much in the spirit of classic space shooters. A lot of them had no music.
I respect your opinion and I'm not saying that music is a bad idea, but the quality of the sfx we do not agree about.