i know nothing about sonar, but if you can't get it to work you could at least built your own metronome by just taking any sample/hit/self-recorded-whatever and copy one next to the other and emphasizing the accents by just making them louder or taking another sample/hit/self-recorded-whatever.

Since you said PC I'm assuming Windows (I don't know much about Sonar). Are you sure your MIDI sound is enabled to play on your sound card and that the volume for MIDI is turned up?

In Windows XP, go to Start --> Control Panel --> (switch to classic view if you need to; I don't work well in the new scheme they use) Sound and Audio Devices. You should be on the VOLUME page. About halfway down will be an area names "Device Volume" with a volume slider and an "Advanced" button below. Click that "Advanced" button, not the one for speaker settings. It will open up the "Master Out" dialog which will contain volume sliders for your computer's audio interfaces. Make sure MIDI is turned up and the "Mute" is unchecked.

If that doesn't work, verify that Sonar is using the correct MIDI device. To do that, access your MIDI setup in Sonar. I don't know how to do that. Unless you have installed another MIDI device, there should be one listed to work with your sound card. I think "Microsoft GS Wavetable Synth" is installed with Windows (I might be wrong, but I've rarely seen a system without it), but I know that CreativeLabs' cards use "Crystal FM Synthesizer" as MIDI output.

In sonar in the metronome settings box, have you tried the audio click, it defaults to MIDI and your m-audio does not have and MIDI component to it.

Ben