FCP7 XML and FCP X XML are completely different. FCP7 cannot read FCP-X XML and vice versa. Media 100 does export FCP7 XML. But there is a tool that can convert between the formats. I haven´t tested it with Media100 XMLs, though.
The 7toX works fine for clips without audio. Good thing that the first program I exported accidentally has no audio or else it wont work at first try. I tried a regular program with audio and wont work at all except if you unsync the entire program. But nothing make any sense anyway. FCX has no export function.
XML is just plain text - you should be able to open it up in a text editor and see if there's anything obviously wrong with the syntax that's messing up the audio for you.
I've never touched Media100 or FCP7 XML exports... I've only ever used XML in a web context, to form playlists for a Windows Media Player Plugin. But it's a markup language, so should just be plain text tags.