I am using a Mac…
Here is a safe way to do it:
Use a microSD adapter that can write lock. Write lock the adapter. Now, there is a hardware barrier that makes sure that nothing can be written onto the SDcard. Insert the read-only SDcard into the Mac. You should see a partition that is called “MOVI Update”, is empty, about 100MB and has FAT filesystem. That’s the intersting partition.
Depending on what filesystem drivers you have installed, you might also see other partitions like “MOVI root” and “MOVI boot”. At this point, unmount the entire SDcard.
If everything went as described above, take the card out, remove the write lock and put your sound file on the “MOVI Update” partition. Unmount the entire SDcard and put it back into MOVI.
Hope that helps,