The issue isn’t so much that Rogen isn’t a great singer, though the actor admits he’s not, the issue is that Disney apparently brought in Pharrell Williams to be Rogen vocal coach, and Rogen’s bad singing is causing Williams problems, which in turn stresses out Rogen.
Seth Rogen has been cast as Pumbaa in the new version of The Lion King coming from Disney.
Playing a role in such an iconic movie is likely stressful enough, but as it turns out Rogen’s singing is the part that is really causing him grief.