Ive had a few furbys that seemed to go through a few minute period where they seem to reset their voice frequency, after...

This ( i think its a 2013) Furby Boom will speak but it makes a quiet crackling noise when it does, then it blacks out...

Try feeding it a bunch till it says its full ("boa dye"), wait a few minutes cause chances are it'll be hungry again so feed it some more Those older ones sure seem to get persnickity and cranky, like a grown up that has low blood sugar or something, let me know if that helps....

Turns out it had a bum motor, (finally my sensetive dog ears came in handy, after sitting there and listening to that clicking, i got a hunch it sounded an awful lot like a record/cd going around really fast but then really slow after that) actually the physical speaker was fine

Have you tried what a bunch of others with mute booms have trie....if the fault is in the speaker itself many have had sucess with poking the speaker membrane actually through the fur & plastic cover.....i have done something kinda similar and i took out the bad speaker and plugged in a better one.....ive found that if none of that works, it may require more surgery to fix/replace a faulty motor....diagnosing what didnt help is oddly enough can be great,MN frustrating but eventually great. Let me know if anything helps

Do you mean the tilt sensor? Like the one that picks up if they're swinging or like flying etc? (I have no idea how to find/fix them on eithet the old ones OR the newer ones, i'll keep a lookout for you though...)