OlSkoolGuy
The Metronome is just another bus (like Master) that feeds its output directly to your sound card/interface to blend in your speakers/headphones with the Master output.
Correction...there are default templates in more recent versions of SONAR that route the metronome to a separate Metronome buss that typically goes to your main interface output. You can re-route this to anything you like (like your project's Master buss...if it has one). It isn't the metronome...it's just what the metronome can be routed to.
What usually has happened to people who have lost their metronome is that they've deleted the soft synth that renders the metronome (i.e. the DXi named GroovePlayer.dll) becuase there doesn't seem to be a way to include it in a project...so what the heck good is it. Find that, or reinstall it and see what happens.
You'll also want to check the routing of your audio metronome in the metronome page under Preferences. The metronome audio out should go to something that eventually makes its way to the output of your audio interface...like your Master buss, Metronome buss or directly to the output itself. (I'd suggest the Metronome buss...'cause then it's easier to mute, automate, change the level or subdivide with a delay in the buss' fx bin.)