Mistletoe Tyrannulet gives a plaintive, monotonous call throughout the day, and may string these calls together into a song. This call typically is transcribed as pyeer or pyeeu, though there is variation and the call may be a two-syllabled peeyup. Both sexes apparently give these calls. The dawn song (presumably given only by males) consists of a series of notes, the first a slow, dropping syllable and the rest short and rising. Skutch (1960) described this song as yer-de-dee, often followed by a trill.
Skutch (1997) indicated that the Mistletoe Tyrannulet's dawn singing bout is longer than that of any other tyrannid in Costa Rica. He also briefly mentions a "song" given at the nest by the female just before a brooding bout, which he described as only a soft murmur with a very low volume.