We only allow built-in converting to AAC on Windows 7.
OGG is a free and open format so we can implement it in Windows XP just fine. However, AAC is a patented licensed format. We cannot encode it to AAC in Construct 2 without having to pay a license which would probably put up the price of Construct 2. The exception is Windows 7 which has a built in AAC encoder which we're allowed to use for free.
To work around it, you should be able to find some free AAC converters (have a quick Google). The final filename should be .m4a. You can then convert the files yourself, and import them to Construct 2.
If you find this annoying, you can blame Microsoft and Apple, who refuse to support the free OGG sound format in Internet Explorer and Safari.