cwm, a word meaning "valley", comes from the Welsh cwm "valley" (pronounced [kum]). It's from Proto-Indo-European *keumbʰ-, an extended form of *keu- "to bend".
The same form became Greek κύμβη kumbē "hollow of a vessel: drinking-cup, bowl". The form κύμβᾰλον kumbalon was applied to the cymbal because of its bowl-like shape. This was borrowed into Latin as cymbalum.