The symbol that looks like a "B" is written as two of the letter "s" in English. I only know this because I took some German in school. So, is Google expecting me to write it as a "B" because it knows I speak English, or should I be writing it as "Strassen," which is German for "Street." Or was I supposed to translate it? D: I tried using the sound button, but it didn't make the sound. :/
Assuming no "Insert Symbol" available I'd try using "B" and it that failed, try the next "word" Captcha offers!
I think they were devised to prove your bona-fide, to try to protect you and the site from spammers.