Does not seem to be recognised during the first boot. Make sure the `alsa-mixer' service is turned on, runlevel `S'. Note that `alsa-mixer' replaces `alsa'.
MP3 sound players are not on the Xubuntu install disk, and they are not in the normal repositories either. Using Synaptic, look under `Settings->Repositories'. Click on the `Installation Media -- Channels' as required, but also be sure to also click on `Installation Media -- Add' and then choose `Community maintained' and `Non-free'.