Stefan Hussong's audio clips come in 3 different versions

ra (low quality)

as RealAudio in low quality:
RealAudio sound clips start playing even before the entire file has been downloaded (streaming)
You need a RealAudio Player such as RealPlayer G2 (download; 2.7 M).
download size (0.5 min of music): ca. 80K

ra (medium quality)

as RealAudio in medium quality:
RealAudio sound clips start playing even before the entire file has been downloaded (streaming)
You need a RealAudio Player such as RealPlayer G2 (download; 2.7 M).
download size (0.5 min of music): ca. 150-300K

mp3 (medium quality)

as MPEG-3 in medium quality:
MPEG-3 (or: MP3) sound clips are compressed according to the Xting-Technology and achieve generally the highest quality as related to file size; depending on the MP3-Player you are using, they also start playing even before the entire file has been downloaded (streaming); in order to keep download size low, you will find medium quality MP3.
You need a MP3-Player such as WinAmp (download; 0.5 M) or MS Media Player (download;4.1 M; plays all audio formats including RealAudio!).
download size (0.5 min of music): ca. 250-350K