Your intro MP3 is probably recorded at a different bitrate than the MP3s your stream is configured to play.  For example, if your stream is configured at 128Kbps, then your intro MP3 and all of your stream MP3s must also be recorded at 128Kbps.

To correct this problem, you'll need to re-encode your intro file at the same bit rate as the MP3s that you plan to stream, or the stream you plan to relay.