Learn Russian: Слушая это аудио, я всегда читаю субтитры. - Listening to this audio, I always read the subtitles.

Слушая is an imperfective verbal adverb, which translates to "while listening." To form it, you take the present tense "they" form of the verb (слушают), drop the final -ут or -ют, and add -я (or -а after certain letters like ж, ш, ч, щ). So here, слуша(ют) + я gives us слушая.

Could I use слушая if someone else was reading the subtitles while I listened?

No. A strict rule in Russian is that the verbal adverb (the -я word) and the main verb must have the same subject. In this sentence, я (I) is performing both actions: listening and reading. If you are listening but someone else is reading, you cannot use this form; you must use a standard clause like Когда я слушаю... (When I listen...).

Why doesn't the ending of аудио change? Isn't it the direct object of "listening"?

Yes, it is the direct object and technically in the accusative case here. However, аудио is a borrowed foreign word ending in a vowel. In Russian, such nouns are "indeclinable," meaning their spelling never changes regardless of the case or number they are in.

Is the comma after аудио strictly necessary?

Yes, absolutely. In Russian punctuation, a phrase built around a verbal adverb (called a деепричастный оборот) must always be separated from the rest of the sentence by commas. This is true whether the phrase is at the beginning, the middle, or the end of the sentence.