I recently bought a rebranded Innos D9 from China. When I tried using the earphones that came with the phone and the Sony earphones I have, the audio is fine. Really awesome too.
However, when I use a generic Samsung earphones, I hear garbled audio, like the sound is coming from a cave. When I play a video, the voices / speech of people are distorted. Also, I don't think equalizers would fix this because I know how they work. I also tested Apple's Earpods, to the same effect.
I would like to know why this happens. Maybe I could download an app to correct the sound, or at least know what earphones to buy in the future.
Note: The phone has a built-in Yamaha music amplifier that boosts AAC or something, if that information would help.
Another update: Found a relevant thread on XDA and the fix I did was to push the answer button on the Samsung earphones, and the audio was fixed. Do note that for the audio to stay that way, the button should be pressed always. This is not a good fix.