v2.3.1

A warning is printed if the minor version (binary) field doesn't match the
minor version in the header (plain text).

Bug fix:

Mixing (pre-amp) level is parsed correctly (xid6).

The length is 4 bytes. The old spec had an error where this field's length was
set to 1 byte. This was fixed in commit 29082c0 but the code was never updated.