Loss of audio

Last weekend i lost the ability to monitor audio through the mini-plug outlet on the back. I still record audio, and the levels are displayed onscreen. But where a mixer is not an option, we are 'deaf' to the quality of audio. I'm not sure it is a physical setting problem (since i have tried most button combinations), but either a software setting (on the menu) that disables the mini-plug output or that the output itself got fried. But before i send it off to the shop, i'd like to troubleshoot any menu settings that may relate to this.
Thanks much,
Swan