Audio player uses MediaElement.js built into WordPress to enable use of HTML5 Audio tag when browser supports it, or to fall back to Flash/Silverlight for older browsers.
Support for different formats changes with different browser versions. So, this list is for latest browsers in use currently. Some older browsers will fall back for some file types.
Supports everything: MP3, OGG, WAV, AAC.
Supports everything: MP3, OGG, WAV, ACC. MP3 support is added with Firefox in version 25.0. Older versions will play MP3 using fallback Flash.
MS Internet Explorer
Supports HTML5 Audio tag since 9.0 and plays: MP3 and AAC. For other formats, fallbacks to Flash and Silverlight.
Supports MP3, WAV and AAC. OGG will not work on MacOS, but it will on Windows if codec is present. In any case, OGG is a problem with Safari.
Version 12 supports OGG and WAV. New versions since 15 support everything Chrome supports.
In most cases, support for different audio formats depend on the device, and even the same OS doesn’t mean that same audio files will work. All mobile devices using Android 2.3 and newer or iOS support HTML5 Audio and play most of the formats.
Android devices can support different formats for Audio tag. iOS and Blackberry devices support MP3 and AAC.
What format to use?
To attempt using direct HTML5 Audio without fallback, best format to use is MP3, it works almost everywhere. For old browsers, fall back will be used in any case.