Phone and sound glitch when headphones are plugged into auxillary port

I recently had my iPhone screen replaced after cracking it significantly. The man who repaired it said he saw my some water damage inside, but my phone had been functioning fine so I didn't think it was a problem. This was about a month and a half ago. For the past few weeks, when I plug headphones into the auxiliary port, the volume will change on its own at random intervals, and sometimes the sound will cut in and out. I have tried turning the device on/off again, blowing in the port to remove any dirt/dust, and rotating the headphone jack, all to no avail. I have checked the pair of headphones with other devices and they are working perfectly so I have concluded that it must be the iPhone.

None of these issues occur when I play music from the iPhone out loud (without anything plugged in the auxiliary port) although the sound is a little gargled (my speakers are sub-par, also might be due to water damage or some other cause).

Does anyone know what might be causing these bizarre glitches? And are there any replacements/repairs possible or do I need a new device?