It's better if you skip the mp3 and play back wav to the the adapter from the computer.
When converting a tap to wav there's usually an alterbative to invert the sound, it depends on how your computer (or mp3 player if you still want to go that route) plays it back. If you get absolutely nothing that could be the problem.
Then there's a matter of adjusting the volume right from the sound source and also to set the head of the datasette to an optimal position.
It's really a bunch of stuff that can go wrong and even if you have done it before it may still require some experimenting.
I also think it's easier to fit a tape adapter if you remove the lid from the datassette.
Skipping to an even easier version would be to get a better interface, like the vimfuego's tape adapter:
It works, try with a very short file first and see if you can get it to load, like a TAP made in VICE with just 0 REM in it or perhaps a head adjustment program that will make it easier to load the next one.
There's a type-in version as well that can help load the first file (at least setting the head at the best position).