You need both .m4a and .ogg to cover all browsers. On Windows 7 and newer Construct 2 automatically encodes the .m4a, but on older versions of Windows it will only encode the .ogg. You must then upload all the files and have your server MIME types set up correctly to serve both kinds of files, and then Chrome for Android and Safari for iOS have some additional limitations about when music can play. This is all described in more detail in the manual.