[Q] No audio, motherboard defect or other hardware issue?

I've had my Skyrocket (I727-R) for little over a year now. Bought it used. For the first seven or so months everything seemed to work fine. I had no issues and was flashing ROMs fine. Around November last year, I attempted listening to music only to realize that I couldn't hear anything from my earbuds. At first I thought it may have been my crappy old earbuds so I tried playing music without the earbuds and making a call. The other side could hear me but I could not hear anything. Since then, I've taken it back to the place I bought it from and they have been co-operative. Usually they charge for diagnostics and such but since I was a recurring customer they did it for free. Anyways, at first, it seemed like they had fixed it by replacing the speaker. They also wiped my SD, flashed the phone back to GB and told me not to root or update the software. I didn't see why it would be an issue however so I used Odin to update to stock JB and root, flashed some ROMs, etc. The audio worked for a day and stopped again while listening to music the next day. I took it back to them and they said it was a motherboard defect and would cost as much as a new phone to replace the motherboard.

I am currently running Dirty Unicorns 4.4.2, 20140331-0105, radio is T989UVMC6 (I'm on Mobilicity's network)

My question is, is there anyway I can be 100% sure it is a motherboard issue? And if it isn't, how can i fix the audio? If it is in fact the motherboard, what is the recommended course of action?
Thanks.