It sounds like damage to the speaker cone inside the device, was it dropped by any chance? Either way you will need to contact whoever you purchased it from to see if any repairs or replacements can be done as the speaker inside this will be proprietary. Without a donor speaker from another unit there's not much more you could do.
If you have no warranty and no replacements / repairs can be done by whoever you purchased it from, it would be worthwhile opening it up and having a look to see if you can spot any physical damage. Then report back here with an update on the issue / provide pictures of the damage.