My iPhone 4 only syncs songs that are AAC?

[downtown]

As the title suggests, my iPhone 4 will only transfer songs that are on my iTunes to my phone if they are AAC versions of the songs. I don't undestand why. If anybody can help me that would be awesome : )

Thanks in advance,
Downtown

EDIT: The songs that are transferred to my phone (the songs in AAC format) will not play at all, but these are the only songs that transfer.