About that missing documentation...

[valzargaming]

JDA/src/main/java/net/dv8tion/jda/internal/audio/AudioPacket.java

Line 44 in 726ef9d

* I've yet to find actual documentation on what the bits inside this value represent.

I've figured out what 0x78 is supposed to be. 0b0111 and 1000. The first 0 is the marker and the payload type 1111000 (or 0x78 & 0x7F = 120, with 0x7F being a bitmask that keeps the lower 7 bits and clears the top (marker) bit).

Per RFC 3551:
Tables 4 and 5 define this profile's static payload type values for the PT field of the RTP data header. In addition, payload type values in the range 96-127 MAY be defined dynamically through a conference control protocol, which is beyond the scope of this document.

...and per opusrtp docs:

120 is the default RTP payload type used by Opus

tl;dr: 0x78 has a marker bit of 0 and a payload type of 120. Because we know Discord uses Opus per the docs I think we can say it's safe to assume that's what these bytes represent.