Way, way back in a time before English had branched off from other Germanic languages, plurals were formed with an –i ending. So mouse was mus, and mice was musi. That plural –i pulled the u forward into umlaut. Later, the –i plural ending disappeared and a whole bunch of other sound changes happened, but we are left with the echo of that mutated vowel in mouse/mice, as well as in foot/feet, tooth/teeth, and other irregular pairs.