These buds are so relatively inexpensive, I don’t know that people really try to fix them–I haven’t seen people post much, if anything, on that. It could be that the wire/cord has gone bad, not uncommon with earphones, or something in the bud itself. You could try jiggling the cord a bit to see if that fixes things–if it does, it would seem to be a cord issue and, I guess, you could cut the cord iff where you think the issue is and splice a new cord, such as from some other dead phones, to the cord.
Personally, I’d just get some new earbuds, if you need them–many, many good options around, at good prices, and with improved sound.
