Long Apple Lossless files sometimes stop playing before the end of the file.
This occurs if the Apple Lossless file is longer than the maximum GarageBand song length of 1999 measures. When an Apple Lossless file is added to a song, the marker does not automatically move past the maximum possible song length to accommodate files that exceed this length.
To resolve this issue, reduce the tempo of the song. Cutting the tempo in half may be a good starting point, depending on the length of the Apple Lossless file.
This document will be updated as more information becomes available.