Great you spoke with developer , and did he not have anything else to say on how to restore the volume? Usually they are more helpful.
About 2 years ago on my Google Nexus Ones at the time
I downloaded an app and lost my ringer volume on both cells the Dev was a deadbeat! No help or did reply to anybody. Checked the sound settings on my cells both were at full volume.
So went to the Google Play Store and found this app fixer for volume problems, installed it and noticed on the app my volume ringer was off. But on the cells it showed full volume on the both N1's cells. And that fixed my problem.
Just one question ,unless I missed it in your post? Did you try a factory reset or not? If not try that and if your volume problem is fixed it was caused by app, if still not working? Then something happened internally within the cell. And hard to say what it is unless I open it up And take a look.
If that does not work I guess revert back since they are willing to do it for you.
In most cases caused by a faulty app, by doing a factory reset will fix the problem, if not the cell itself is the cause. Then bring it in for repair.