It's not the TV, the sound is screwed up because the region is PAL. I think the only way to avoid this was to region change.
Thanks for replying.
Should I try AnyRegion Changer for that? I'd rather avoid this since it can brick my Wii.
EDIT (SOLUTION!):
Today I ran AnyRegion Changer and changed my game region to JP. However, this did not solve the horrible sound quality. By the way, I even noticed the game was lagging all the time, too. It just did not seem that way because Japanese text still was faster than English.
Thus, when changing my region back to European, I noticed there was an option to alter your TV video mode, which was set to PAL and could be changed to either NTSC or MPAL. I figured this might be the problem after reading an article about PAL and NTSC TVs. So I changed my TV setting to NTSC, and it solved the problem.
A small warning, though: I believe it's possible that some PAL TVs do not understand NTSC input. So, be aware that changing your video mode in AnyRegion Changer may cause your TV to do all sorts of weird stuff, so that you will not be able to see what you're doing on your Wii anymore!