Most burning tools that support the creation of audio CDs support only the most common formats by default. Those formats are normally mp3, wav or wma. Some suits support other formats as well but I never found a burning suit that supported all of the formats that I needed. One way to solve this issue would be to use plugins that would increase the supported formats, one plugin pack for Nero can be downloaded here. (Click on Nero Audio-Plug-Ins..)