The masculine version of the adjective "nervioso" was used, but the recorded voice was obviously feminine. It should match the gender of the speaker - "nerviosa".
Interesting! It never occurred to me that the choice of voice was also randomized by the system. I might assume that Duolingo's architecture doesn't support altering the exercise based on the perceived gender of the randomly chosen voice, so I won't hold my breath 'til this is fixed. :)
The only silly question is the one never asked.
"Yo" is this sentence is optional. "Estoy muy nervioso ahora." is quite acceptable. The subject pronoun is used for clarification or emphasis. In this case, "estoy" can only be translated as "I am", therefore the "Yo" isn't needed. But if you were to say "Está muy nervioso hoy", that could mean either "You are very nervious" or "he is very nervous". Therefore the "Usted" in "Usted está muy nervioso" clarifies that it is "You" that is very nervous, and the "Él" in "Él está muy nervioso" clarifies that it is "He" that is very nervous". (The same for "she" if you change "nervioso" to "nerviosa".)
"Yo soy muy nervioso ahora." is incorrect with or without the "yo". "Nervioso" is a descriptive condition, much like "enfermo" (sick), "triste" (sad), "cansado" (tired), "feliz" (happy), etc. These all require the verb "estar" rather than "ser".
A native Spanish speaker please comment. My understanding is that when the gender is not specified, then you go with the masculine. In this case, the sentence given was "Yo estoy muy nervioso ahora." If it were a written sentence, then no problema. In this case, the sentence was spoken. If spoken by a male voice, otra vez, no problema. However it was spoken by a female voice. So my question is this. Should it be "nervioso" because the subject is "I" which is not necessarily gender specific, or should be it "nerviosa" because the speaker is female?