to speak: to open one's mouth and produce speech, with the focus on the act and activity of speaking itself
This is the New Testament's primary verb for the physical act of producing speech. It points to the activity of talking, to vocalization happening, rather than to the specific content of what is said. When someone is healed of muteness and speaks (Matthew 9:33), when a crowd falls silent and then reports what they heard, when the Spirit enables utterance in worship (Acts 2:4), this word is at home because it names the utterance itself.
This accent on the act, rather than the content, is what sets it apart from its closest companion, λέγω. That verb is the standard vehicle for introducing what someone actually said, answering the question "what did they say?" This word asks instead "was speech happening?" So it is the natural choice when the fact of speaking matters: a mute given voice, a crowd noting that no one has ever spoken like this man (John 7:46), an angel delivering a message, prophets described as having opened their mouths by the Spirit (2 Peter 1:21).
When the author of Hebrews writes that God spoke to the ancestors through the prophets and has now spoken through a Son (Hebrews 1:1-2), the word chosen is this one: the accent falls on the event of divine speech, the sheer fact that God gave voice. The specific content of those words is what λόγος names; this word names the speaking.
In 1 Corinthians 14, where Paul discusses utterance during worship at length, this word dominates precisely because the debate turns on the act and manner of uttering rather than the content of what is said. Speaking in a tongue and speaking in an intelligible language are both instances of this verb, and the question Paul asks is whether the utterance builds the community.