Who else would like to have audio translations to do? They could have each sentence be a separate Spanish audio file that would play, and then we could type the English translation. That could be used as the English transcript or subtitles for the Spanish radio or TV program it comes from.
I think that this is really needed. Understanding syntax and formulating sentences is one part of learning a language, but the other very important part is developing an understanding to be able to decipher spoken words.
A great solution to this problem would just be to add a "transcription" page, where people transcribe audio into the written language. This would only require copying the current "translation" document pages and substituting the original text part for an audio file. After the audio is transcribed, you can simply re-import the page into the "translations" part of the platform.
Just as you plan to sell the translations in the future, you could sell transcriptions and translations of audio files. You may even be able to sell this service to youtube; they have an automated transcriber but it is not very accurate.