IDEA encryption is substantially faster and generally considered to be much more secure than DES encryption. However, IDEA is newer, has not been formally adopted by governments, and is patented, restricting its commercial use. If your CPU is fast enough, you can enable any combination of IDEA, DES, and key file encryption. But since PGP uses IDEA to transmit message bodies, if you rely on PGP to exchange keys with other parties, the fundamental security of your voice link rests upon IDEA alone.
Cipher block chaining is used within each sound packet, but not from packet to packet. If that were done, loss of a single packet would render the entire rest of the conversation unintelligible.
Speak Freely will continue to correctly receive unencrypted sound from a given host even if an IDEA key is specified for the connection.
The IDEA encryption algorithm is patented and may not be used commercially without a license; see "Patent issues" for further details.