AAC decoding is device dependent in Android market. It means some devices will do it easily, some will struggle, some won't read it at all. You can read here for reference.
A couple of things to consider:
You can test for device capabilities to playback AAC audio:
var a = document.createElement('audio');
var aacSupport = a.canPlayType('audio/mp4; codecs="mp4a.40.2"');
console.log(aacSupport);
If aacSupport returns probably it is ok to use AAC. If it returns an empty string it is not ok. Otherwise you could use mp3 which has larger support amongst device. Try it in your case and see if the devices struggling with the AAC playback can play mp3 fine.
Also you can remove the type attribute in your audio tag it is only useful in a child source tag. Not sure audio/aac is a valid MIME/TYPE anyway.