Most of my time with Final Fantasy Record Keeper has been, unsurprisingly, with the sound muted. It happens with smartphone games. I know this, and yet I still went through the trouble to ensure mine had awesome music implemented well. FFRK does the first part (It's Final Fucking Fantasy, after all.) but manages to fail on the second. I had my headphones in already when I decided to spend some time grinding out some Magic Stones so I can unlock White Mage Lenna. After about ten seconds of the totally awesome battle theme from FF5 it looped. And by "looped" I mean it started over. And then played a bit. And then I finished the battle. And then the victory fanfare played. For a few seconds. Then it started over again. Then the music stopped playing for a bit.
So something is not right. Which is a shame and yet not something I think they'll fix any time soon since I have to imagine there are other bugs and issues much higher in the "To Fix" list. Just today all players got 1 free Mythril because the Jump command was not working properly and they just fixed it.
(If you selected a specific target for the attack it would still attack any target like an unspecified attack would. This is a fairly neat idea for the game. You can just select attack and it will pick the target for you. Or you can tap on the target before selecting attack or an ability and then it will be used on that target. Except for Jump. Well, until today.)