Ah yes, no sound with Flash nonfree is a common issue. Unfortunately I can't say the solutions are always the same, but there are quite a number of solutions/workarounds out there. NB: If you try installing flashplugin-nonfree-extrasound, it may be worth grabbing libasound2-plugins and libasound2 as well.

Hmm, did you get any useful output in the cups logs when you tried the canon driver?