ak4497eq has a LDO regulator inside. That LDO is only for digital internal circuits ? Or is it for analague circuits too ?
The LDO is just for the digital core and can be bypassed. However, I can’t say I’ve noticed a difference by bypassing it with a good external supply.
Whether or not the a good external supply makes an audible difference may depend on the quality of the rest of the implementation. AKM certainly used an external LDO for AK4499 evaluation board. On the other other hand, AK4137 data sheet recommends not to use an external LDO in some cases. So, it may depend on the particular chip too.
AKM doesn't say what the exact limitations of the internal LDO might be. In cases where the data sheet does not give advice on the subject its up to the designer to try an external regulator or use the internal one.