A B737-700 CREW RPTS THE 'ON RWY XX' RAAS VOCAL ANNUNCIATION INTERFERED WITH A CLRNC AMENDMENT AS THE ACFT WAS ENTERING THE RWY FOR TKOF.

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WE WERE GIVEN CLRNC; 'ACR X CLRED TO CROSS RWY 19L; TAXI INTO POSITION RWY 19R.' WE COMPLIED WITH THE CLRNC. ONCE IN POSITION ON RWY 19R; WE WERE GIVEN; 'ACR X CLRED FOR TKOF RWY 19R; MAINTAIN 4000 FT ON DEP.' (THIS WAS A CHANGE TO OUR CLRNC ISSUED 5000 FT). AT THE EXACT MOMENT THIS CLRNC WAS ISSUED TO US BY SNA TOWER; THE RAA SYSTEM VERBALIZED; 'ON RWY 19R.' BOTH THE RAAS VOICE AND THE TWR VOICE WERE FEMALE AND THEY BOTH WERE OF NEARLY IDENTICAL CADENCE AND MODULATION. FORTUNATELY; THE FO HEARD THE ALT CHANGE BY TWR. I DID NOT. HAD WE BOTH MISSED THE CLRNC CHANGE AND TWR MISSED OUR READBACK; DUE TO ATC RADIO CONGESTION (WHICH HAPPENS FREQUENTLY); IT COULD HAVE RESULTED IN AN UNSAFE EVENT AFTER DEP. I REALIZE THE RAA SYSTEM IS NEW AND HAS BEEN PUT IN PLACE TO ENHANCE SAFETY; BUT CLEARLY THIS EVENT DEMONSTRATES HOW IT CAN ADVERSELY AFFECT SAFETY. IT IS MY BELIEF THAT THIS SYSTEM SHOULD ONLY VERBALIZE YOUR SITUATION WHEN IT SENSES YOUR ACFT IS IN A POSITION THAT IS IN CONFLICT WITH YOUR CLRNC. IF I AM INCORRECTLY LINED UP ON RWY 19L AND THE ACTIVE RWY IS RWY 19R; IT SHOULD THEN AND ONLY THEN TELL ME THAT I AM NOT IN COMPLIANCE. IF I'M GIVEN CLRNC TO LAND ON RWY 16R AT SEA AND I HAVE THE RWY 16R LOCALIZER TUNED AND I SUBSEQUENTLY LINE UP ON RWY 16L; IT SHOULD VERBALIZE 'CHK RWY' OR SOMETHING SIMILAR.