AAC support is good, and the phone can only properly handle LC AAC. HE-AAC (a.k.a. AAC+, a.k.a. eAAC) will play since HE-AAC is designed to be backwards compatible, however the SBR component will be ignored. The same is true with HE-AAC v2 (a.k.a. eAAC+), only the normal LC component of the file will be played. The SBR and parametric stereo (introduced with HE-AAC v2) is ignored. Since HE-AAC v2 encodes the sound as a mono-aural track and the stereo separation is recreated through the PS data, HE-AAC v2 files played on a K750 actually play back in mono.