Problem with audio output on OS X 10.10.4
While I've never experienced this issue in Yosemite, I did have issues with audio output on an older OS. If I remember correctly, my solution was to reset the core audio.

The following is reason to believe some have experienced loss of audio in 10.9: http://www.imore.com/losing-audio-os-x-mavericks-heres-what-you-can-do-about-it

This thread shows a means of automating the process, in case that solves your problem: http://forums.macrumors.com/threads/create-a-service-to-reboot-core-audio.1697918/