Big shout out to this person from the Skeleton Clique for uncovering hidden vocals in two twenty one pilots songs! They both sound very eerie and I think the deep voice in 'Cut My Lip' is Tyler.
...and no, it's sadly not a clue to the next album. Hear the hidden vocals for 'Cut My Lip' here . Check out the hidden vocals for 'Legend' here. Take a listen to the album version of the song and see if you can hear it!