Speak, talk, converse can all mean to articulate words so as to express one’s thoughts.
Speak is, in general, the broad term and may refer to utterances of any kind, however coherent or however broken or disconnected, and with or without reference to a hearer or hearers.
Talk, on the other hand, usually implies an auditor or auditors and connected colloquy or discourse. But speak is also used of relatively weighty or formal speech (often public speech), and talk, of what is more or less empty or frivolous.
Converse implies an interchange in talk of thoughts and opinions.