Why is is only one side of the audio playing

You may need to clean the headphone jack. It may have lint or dirt built up on the point that makes contact with the headphones plug. Which may result in a poor connection causing only one side of the headphones to work. Try blowing out the headphone jack with compressed air or clean the jack with 90% or above isopropyl alcohol and a q-tip or pipe cleaner or whatever small brush can fit in the jack. It could also be that the headphone jack is not making proper contact with the contacts on the logic board. You would have to open the phone to be able to see if this is the issue. If cleaning the jack doesn't fix the issue then the headphone jack may be faulty and needs replaced. There may also be a software or app issue causing the problem, which a hard reset may fix. Let me know if you figure it out.