My son wanted to play some games so I hooked up out a really nice set of Monsoon speakers I had in storage from an old PC to the ol' 1200. I used an RCA-to-mini cable with a gender changer from radio shack to connect the speakers. We started playing ThemePark AGA and my son quickly commented that the sound was really sloppy and distorted. I tried some demos and he said the same thing.
Those speakers do have a built in amplifier right?? If not, then the problem could be as simple as the amiga audio ouput not having enough juice to power those speakers. If this is the case, then unhook them fast, or you could blow the audio output on the amiga.
If you have a regular stereo amplifier, then you could just try to hook it up to that and see if it makes any change.