@pokey: If the IAudio 7 uses the standard USB Mass Storage driver, it would work in the same way as a flash drive. However, the catch is that the N8x0's USB port is OTG (on-the-go, e.g. switchable between host and peripheral modes), not technically a USB host; thus, it can't supply as much current to the connected device as your PC can. The implications of this are that only low-consumption devices will work, unless you run them through a powered USB hub. (Which would require a power outlet, or a compatible portable battery pack (IOGear makes some things like that, if you're interested)). What is happening (as far as I can tell) is that your audio player is expecting the N810 to charge it, and since it can't, it doesn't work. I've never used an IAudio 7, but my T2 has an option in the settings (under General, I think) to turn off charging when connected to a computer. Turning that off should make it work, if anything will, but I can't guarantee it. Do note that by turning that option off, you will be unable to charge the player from a computer, but only from a standalone USB charger.
I hope that helps...

P.S. Also make sure that if there is an option for MTP/UMS modes, that it is set to UMS - the N810 does not support MTP transfers.