mp4 playback is skipped

I have a playlist consisting of mp3 and mp4 audio tracks (the mp4 tracks were downloaded on iTunes and are encrypted). When the playlist goes from an mp3 song to an mp4 song, I get an error message and the mp4 track is skipped. The weird thing is that if I just double-click on the mp4 track, it will play, and if I have two mp4 tracks in a row, it will only skip the first one.

Songbird doesn't tell me there are any updates available, and I have the Quicktime add-on installed. It seems that when Songbird comes to an mp4, it decides to skip that song first, then play whatever is next (even if it is another mp4). It always gives two error messages, which are copied below.

Error: GStreamer error: The stream is encrypted and decryption is not supported. Additional information: qtdemux.c(3720): qtdemux_parse_trak (): /player/uridecodebin3/decodebin23/qtdemux1
Error: GStreamer error: This file contains no playable streams. Additional information: qtdemux.c(1936): gst_qtdemux_loop (): /player/uridecodebin3/decodebin23/qtdemux1:
no known streams found

Otherwise, SB has been great for me and I really like using it. Thanks for the help!