What do the characters symbolize in Samuel Beckett's play Words and Music?

Words and Music is seemingly a play that is ultimately about the creative process. The play truly consists of three "characters," though the word can only be used loosely, since the play is entirely aural, and one of the "characters" is literal music (sometimes referred to as Bob).
The other two characters are Croak and Words (spoken poetic words that are sometimes referred to as a character named Joe). Once again, since the play is a radio play and is an aural experience, there is no physical manifestation of any of the characters.
Croak is a symbol of the artist, any artist, perhaps Beckett himself. Frequently referred to as "my lord" by Words, it can be surmised that Croak is the master, ruler, or creator of Words.
Words, quite simply, symbolizes language. Beckett, being a poet and playwright, frequently used words and consequently needed to gather an understanding of them. Croak attempts to command Words, to no true avail.
Music, then, symbolizes music, or another aspect of the creative process. While Words attempts to convey a thought, Music's interpretation of the same thought is often conflicting. It's worth noting that the musical numbers for the radio play were composed entirely by John Beckett (who is, quite interestingly, Samuel Beckett's cousin). Thus, Music can be seen as an alternate form of the artistic expression, or even the viewpoint of another artist.