The data sheet for this part could tell more, it is not much help...
Have you tried this? From the data sheet:
P
ass Through Mode
In cases where the host processor needs to communicate directly with the SpeakJet,
the TTS256 offers a pass though mode that will send any received codes from the
host directly to the SpeakJet without translation of any kind.
This is accomplished by sending the text "passthruon" and an end-of-line character.
All characters sent to the TTS256 will be sent directly to the SpeakJet without
translation. An "X" ($58) will terminate the pass through mode. Note that this
character cannot be used as the second byte of a two-byte command as it will
terminate the pass through mode. An "X" is also used to terminate the SCP mode for
communication directly with the SpeakJet internal registers. An alternate \A may be
used to terminate SCP mode and not pass through.