Is there any place where I can post questions about non-computer hardware issues? To my knowledge, Super User is specifically about computers. Is there a Stack Exchange site like Super User but for hardware? Example would be: a set of speakers that's malfunctioning.
At present, No
However, you can ask the more specific and expanded version of this question in the Meta site of Musical Practice and Theory SE. According to their Help-Center:
usage of specific music software or hardware
is on-topic. So, a clarified version of your question might be on-topic there. So, maybe you can go ahead and raise a query in their meta site.