During which selector are you making the call, and what would you like to use that info for?
On exSelDoExport, you will get a exDoExportRec.exportVideo and exportAudio, to indicate what to export.
I'd like to use them on exSelExport, but i think exporting video/audio and knowing that whether the source file has video/audio or not are different matter.
I've tried, but it seems kExportInfo_SourceHasVideo and kExportInfo_SourceHasAudio doesn't work at all.
I've reproduced the problem and logged this as bug #3058501. Thanks for pointing this out!