Synonyms for speak

verb

1. To utter

   vocalize, pronounce, express; see utter.

2. To communicate

   talk, converse, discourse, articulate, chat; see also talk 1.

3. To deliver a speech

   lecture, declaim, deliver; see address 2.

speak and talk are generally synonymous, but speak often connotes formal address to an auditor or audience who will speak at the dinner? and talk often suggests informal colloquial conversation we were talking at dinner; converse suggests a talking together by two or more people so as to exchange ideas and information the leaders of both countries met to converse for several hours; discourse suggests a somewhat formal, detailed, extensive talking to another or others she was discoursing to us on Keats

so to speak

to speak of