intransitive verb.
To hear (speech or someone speaking) without the speaker's awareness or intent.
intransitive verb.
To hear something without the speaker's awareness or intent.
The Century Dictionary and Cyclopedia
To hear (one who does not wish to be heard or does not know that he is heard, or what is not addressed to the hearer or is not intended to be heard by him); hear by accident or stratagem.
To hear over again; hear from beginning to end.
the GNU version of the Collaborative International Dictionary of English
transitive verb.
To hear more of (anything) than was intended to be heard; to hear by accident or artifice.