My Nexus 4 recently started exhibiting an annoying bug: no audio in phone calls. Sometimes I could hear the other person but they couldn’t hear me, sometimes vice versa. Sometimes neither. Rebooting fixed it, but only for a little while.

A few searches led me to these two bug reports, and to this workaround: open Settings, Developer, go to the Media section, and disable NuPlayer. Works for me so far!