Since Yosemite 10.10.1 (if it's at all related with OS X) I have sound problem. Firstly, hardware. Logitech Z906 speakers are connected to digital audio out on Mac mini. Sometimes when I turn it on, volume level is very low, about 2.5 - 3 times lower. Reset does not help, only reset with clear PRAM helps. Because you don't have OS X volume control, volume level is controlled by Z906 console or remote. I do not touch it, just reboot with clear PRAM, and volume level is almost 3 times higher. Any thoughts ?