First of all, none of the GBA bios audio is chiptune, *is it all made with the direct sound channels*
Second, as far I know, there is no raw audio sample in the BIOS in the sappy format. If the samples are there (and they certainly are, unless they are computed "mathematically" which sounds unlikely), they are compressed.
And I know there is a mixing engine in the BIOS, which is used for the jingles and that can be used for games as well (how many games actually uses this ?) but I don't know if there is a (compressed/encrypted) sappy engine in the BIOS.