assuming you checked the system volume on your Mac... .. I'd check the Audio MIDI setup app and make sure your channels aren't turned down for some reason. Another possibility is that an app is suppressing your volume. For example, if I am watching a video and then flip to Home.app to check the baby monitor, if I load the monitor video, the Mac sound will be turned down (not off, just way down) in order to let the monitor audio come to the forefront. So maybe something like that is in play.