Vsotvep’s comment about Japanese wigo becoming igo got me thinking about the loss of sounds in languages.
Japanese has lost the syllables ye, we , and wu.
In English we lost the ability to pronounce the k in words like knife, knight, know, and knee.
According to a linguistic friend of mine, many speakers of Classical Latin didn’t pronounce h, or pronounced it inconsistently. Since h is pretty prevalent in Latin words, eg. harena (sand), it was probably in the process of being lost.
According to the same guy, Classical speakers didn’t pronounce r in words like carmen and carta; since r is written, we can suspect that it was lost here in Latin. Some diphthongs were becoming rare in the transition from Old to Classical Latin, like oi, eg. oinos --> unus.
What sounds were lost in your languages?