There aren't any viruses for the Mac.

And there's no malware that makes a bell sound. Malware doesn't tend to want to call attention to itself.

If it's a single bell, you're likely hearing the sound of incoming e-mail. This would be caused by you not actually quitting your e-mail program but just closing it instead (a common switcher mistake).

You can hit command-tab to see what programs are actually open -- or just look down at the dock. The ones with "lights" underneath them are open.