ATRAC3plus - is an encrypted ATRAC3
Yes, I confirm that almost all the files for the PSP are encrypted in ATRAC3plus, in particular, these are system files like gameboot.pmf - a screensaver when the game starts. And many games for PSP also contain a sound in the format ATRAC3plus. And if we can play PSP games on PS3, then PS3 can also play this codec.
But the truth is a bit different, because the PSX games are played through the PSX emulator, both on the PSP and on the PS3. Therefore, I have not yet tested this case on the PS3, you just need to censor the sound and check.
Also, PSP has backward compatibility. For example, if the game had the sound ATRAC3plus, then you can change it to a normal ATRAC3 with the header
RIFF .... WAVEfmt, then the PSP will still play it. The sound is decoded by a hardware decoder.