https://www.scribd.com/doc/24949816/...-Communication
According to the diagrams in this file (page 227, and on), with the park brake not set and the CVR test button pushed, the CVR gets the command to generate the test audio, but it doesn't get to the speakers because a park brake relay interrupts the signal. This suggests the test tone is recorded.
Pushing the test button also stops TCAS audio reaching the speakers.
Bizarre