From the looks of your board, there looks like battery acid repairs were done in the area around J21, where the ribbon cable between the boards connects to the MPU. The music test is intended to test the sound card, but in order to do it properly, it has to be able to communicate through U42 on the MPU board to J21. The video you posted of your sound test, I am pretty sure you are not just missing the one sound, I believe it is playing the wrong sounds in a couple of those tests at well. Perhaps someone who owns a Taxi can confirm which sounds are incorrect.
Regardless, from the pictures you posted, it appears that SRC3 has been replaced. It lies along the communication lines that runs between U42 on the MPU board and J21. If you have a break in one of these lines, your sound board will do funny things like play the incorrect sounds during the music test. You may want to check the continuity on all the traces I have circled in red.
edit: You may want to also check all of the connections from J21. The rest pass through SRC1, and there are a couple that come from U41.