Imported sound loops unexpectedly when pasted into AS3

I am creating an animated music video in Animate that requires lip syncing. I built it initially in an older version of Animate but now have been using CC. I lip synced it to the music which exists on a layer within the file. When I realized that the parenting option was not available in that file format I copied and pasted everything into a new AS3 file. Everything transferred perfectly except the music now has 758 frames of intro percussion instead of 302 frames of intro percussion. The speed of the music has not been affected. It is like the program has added more measures of the lead in music. I have deleted and brought in the music fresh but it still has 758 frames before the singing starts instead of 302 frames. I am not sure why/how the intro would now be longer as everything on the time frame looks exactly like it did in the previous file. And also as a result, the end of the song is cut off, while the song in the previous file ends as it should within that 2.5 minute time frame. Does this ring any bells for anyone?