Your songs are probably not mono, or are too large.
You can convert to mono by going into the Audiotrack settings if your using Audacity which I recommend.
First you need to split the left and right tracks by hitting the "Split Sterno Track", then select the track you want and hit "Mono". This will convert your song to mono.
If you want both tracks together, you may need to use Project>Quick Mix. This will mix both tracks once split.
Otherwise, the files are too big and are being ignored. If they are too big, just loop them shorter.
