Audio codec support has been a thorn in our side for years. It was a problem when we first got going in 2011, and six years later, browser support is not a whole lot better. Construct 2 still uses two audio formats to support all browsers, MPEG-4 AAC (.m4a) and Ogg Vorbis (.ogg). AAC is a patent-encumbered format and distributing encoders incurs a fee, which would make the Free Edition uneconomical.
Read more at Scirra