Could be that one of the channels in the amplifier is dead, or one of the speakers has blown out due to excessive volume (those tiny speakers sure do push a lot of sound).
To check and see whether this has happened, just follow the beginning part of this video on how to remove the casing. Then you can visually inspect the speakers for damage, and also check them with a multimeter.
By playing music through the speaker, you should be able to see which of the speakers isn’t working. Then you can disconnect and pull the speaker out and check the resistance with a multimeter set to the 200Ω or continuity setting. It should show a value around 4-8Ω if the speaker is good, 0 of it’s shorted internally, and and an “open circuit”, “1” or whatever it normally shows when you don’t touch the leads to each other.
If the resistance is in the 4-8Ω range, that means there’s something wrong electrically with the control board. Most likely a dead amplifier channel, since you said it happened even with an aux cord.