Two main uses: (1) baby talk - almost always in the set phrase 咿呀学语 (yīyā xuéyǔ, "to babble and learn to speak"); (2) the creaking of an oar, door hinge, or rocking boat. Both build on the high front "yi-ya" sound - the very first vowels a baby produces, and the same pitch as wood under stress.
memory
Both characters carry the 口 mouth radical - a strong visual cue that this word is a sound.
