Yes, you can save the mp3 files to the Tyros 5 hard disk and you can play them in the Audio player. (p 78 of the Owner's Manual).
Tyros 5 can play mp3 files, but should the mp3 files not be recognized by Tyros than they can be modified and saved with different settings using a program like Audacity (free download).
Or, you can use Audacity to convert them to .wav files, but then they will take up a lot more space.
You can set up the keyboard sounds and save that setup, together with the (opened) mp3 to a Registration bank button.
Registration banks are the ideal way to save your preferred settings and you can then recall them at the push of a button.
You can make and save thousands of Registration banks to the hard disk.
Here is a link to some tutorials on Registration banks made by Fred Smith:
https://www.psrtutorial.com/lessons/playing/regist/05regVideo.htmlStijn