It should work fine for you. I have the EXACT same setup on "2" 755T's and I have tried the FULL 2.16 gig maps on SD with no problem. I also have US & CAN only on one of the units.
I'd say check your spelling by right clicking the gmasupp.img on SD and selecting properties from the menu. This will insure sure you've not inadvertently named it "gmapsupp.img.img" which is actually a common problem if your OS is hiding file extensions by default.
It seems like it's got to be some sort of naming isssue or a faulty IMG/map file. The card interface is operational aty least to some extent or the MP3's wouldn't be playing.
Another quick way to check the SD is to put any JPG on the SD and you should be able to see it with the 755 photo viewer. I suspect this will simply work since your MP3's work.
Out of curiousity... what is the firmware version of the 755 update installed ?
I have V4.5 and as far as I know that is the latest 755 firmware.
If you have a .unl or .gma files in the garmin folder of the 755 you might try temporarly removing/renaming them
Also...
are you using the JCV and 3D buildings?
The following SD setup should definitely work for a 755t
sd:\garmin\gmapsupp.img
sd:\garmin\gmap3d1.img <---- must also be unlocked to use
sd:\garmin\JCV\D1279220A.JCV
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