An updated version of the libogc example oggplayer to support multiple voices playing simultaneously. All functions now take an additional s32 voice argument (pass 0 to match behaviour with the original), and the maximum supported voices can be configured by changing the OGG_VOICE_COUNT macro in the header file, from its default of 4. When playing audio from other sources, ensure the voice you play on is equal to OGG_VOICE_COUNT or higher.
If your intellisense keeps showing errors in oggplayer.c and not recognising OGG_VOICE_COUNT as existing, it's probably because it is using the version of the file from the devkitPro examples folder. Either change the C/C++ extension include paths to not include that folder, or delete the examples.