Hi Stilezzkid,
Since updating to iOS 14.6, we understand songs stop playing after 15 seconds. We'd be glad to help. Let's make sure to restart your iPhone if you haven't done so already. Sometimes a restart can help resolve issues.
You can also try removing the song and adding it back again to see if you're able to play it.
if the issue continues try some of these steps:
Turn off and turn back on Sync Library. Go to Settings > Music > Sync Library to do so. Large libraries may take some time to sync back up. Also, go to Settings > Apple ID > Media & Purchases, sign out and sign back in. Note that offline songs may need to be redownloaded again.
As the two final steps testing after each one, go to Settings > Music > Audio Quality > Lossless and turn this off. Test and do the same for Dolby Atmos by going to Settings > Music > Audio > Dolby Atmos.
Let us know if any of these steps help.
Take care.