Sound, wireless-n, for the rest it is mostly at least comparable. If you compare specs there is almost nothing else that jumps off the page.
Just newer (and better?), sound and wireless-n. Oh, and the placement of the cable-connectors is much better on the 930. It is quite a hassle to connect/disconnect the 920 with the sockets at the base.
I have seen, do not own myself, a network with both, and I could not see any noticeable differences.
Just my $0.02