Learn French: Nous travaillons dur sur l'écho et la basse. - We work hard on the echo and the bass.

In certain fixed expressions, French uses an adjective as an adverb. When this happens, the word becomes invariable (it never changes form). Travailler dur is the standard expression for "to work hard". If you used the actual adverb durement, it would change the meaning to working "harshly" or "severely" rather than putting in a lot of effort.

If the subject were feminine, like Elles, would dur change to agree with it?

No, it remains exactly the same. Because dur is acting as an adverb modifying the verb travailler, it is completely invariable. It will always stay dur regardless of who is doing the work.

Is sur the standard preposition for working "on" something?

Yes, when you are talking about putting effort into a project, task, or a specific element like the audio in this sentence, travailler sur is the natural and correct choice.

Could la basse be confused with other similar-sounding words?

Yes, it is easy to confuse with la base (the base or foundation) and le bas (the bottom). Here, la basse refers specifically to the musical bass, meaning either the bass frequencies or the bass guitar.