Definitely an issue with the phone.
I have had the same volume issue with my maxx. phone packing date 8/23/13.
Volume was great the first day on phone calls and speakerphone. Then, yesterday the headset volume (at max) was insanely low on calls. It put it on speakerphone (max volume) but could still only make out parts of my call. I rebooted once. I ripped off the otterbox commuter mid-conversation thinking it was covering part of the earpiece speaker. No change. I toggled airplane mode, no change. I DID notice that exactly where I held my ear made a difference--if I didn't hold my ear just over the one thin speaker hole, I would have zero volume.
Then I power cycled a second time. Instantly, symptoms went away. I had to lower earpiece volume quite a bit, and when set loudest it sounds like the person is shouting at me. I put the call on speakerphone and everyone in the car complained it was so loud. When the issue was occurring, the speakerphone call was barely noticeable. In fact, I had to hold the phone an inch or two away from my ear to make out any part of the call .