With your Android mounted in Ubuntu, use your favorite text editor to create a hidden file called .is_audio_player in your device’s main directory. The dot at the beginning and lack of file extension at the end are both very important!
In Rhythmbox, you’ll see your Android’s storage on the left side under Devices. When you copy audio from your computer to your device, Rhythmbox automatically “transcodes” that audio into a format which Android’s music software understands. Transcoding doesn’t alter the audio files on your computer, but instead creates special versions “on the fly” for your device. With normal settings, Rhythmbox will send audio to your Android as 128 Kbps MP3s. If you’d prefer higher quality audio for your device, stay in Rhythmbox and go Edit => Preferences => Music tab => Edit button => CD Quality, MP3 => Edit button => then alter the text to obtain the quality you desire. To assist you, the text that I use to transcode from my computer to 256 Kbps MP3s on my Motorola Droid is below.
audio/x-raw-int,rate=44100,channels=2 ! lame name=enc mode=0 quality=0 bitrate=256 ! id3v2mux
EDIT: You’ll probably need to activate the Medibuntu repository using these instructions, then use Synaptic Package Manager or the command line to install ubuntu-restricted-extras.