optimal audio (for video) for web videos

That's an open ended question. Talking head may be mono in a lower bit rate. A classical concert video may be stereo at a bit rate the audiophile will appreciate. One might look at a good music encode from the iTunes store to get an idea about the upper end. The lower end of acceptance might be self evident listening through your computer speakers using both cheap and very good computer speakers to compare.
The web is a very big place with some people on the lowest end ADSL connections and others with high speed "Fiberlike" data connections.