xfce4-mixer seems to use the standard "%configure" in the specfile. This makes the configure script autodetect how to build the program, and it seems to pick OSS. Given that fc6 and above come ALSA-enabled, this should probably be changed. This trivial patch seemed to work on a brief build and test...
Created attachment 154370 [details] specfile patch -- build the program for the ALSA backend
Bummer. I thought I fixed this a while back. ;( Looks like it might also need BuildRequires: alsa-lib-devel and some other tweaks. Will do some testing here. Thanks for the bug report.
xfce4-mixer-4.4.1-2.fc7 has been pushed to the Fedora 7 stable repository. If problems still persist, please make note of it in this bug report.