1.   He paced back and forth, muttering things I could barely make out.

2.   He still mutters nasty things about his own coach, his teammates, the opponents, the referees, the fans, the universe in general.

3.   Star player leaves in a huff, muttering nasty things about the organization.

4.   The officer walked away to note some information and my husband muttered incoherent things under his breath.