AAC (Advanced Audio Coding) is a lossy audio format used as default for Apple products. Thus it can be easily found on the internet, although it is not as widely supported as MP3.
Free M4a to MP3 Converter is able to convert AAC to MP3. Alternatively you can choose to convert AAC to WAV, if you intend to convert it further into another format, like OGG Vorbis. If you get errors when trying to convert AAC files, check if they are really AAC. Sometimes wrong file extensions may be used.
The process of converting is quite simple. Just drag and drop your AAC files into the program window and follow steps described in the “Quick Start” section. Set custom output folder, if needed. For MP3 format, additional settings are available. You can preview AAC files by playing them.
It is also possible to convert entire folders of AAC files to MP3 or WAV.