Learn Spanish (Latin America): La pronunciación va variando notablemente. - The pronunciation is gradually varying noticeably.

Using ir + a gerund (va variando) highlights that the action is happening gradually, little by little, or step by step. If you just said está variando (estar + gerund), it would simply mean "is varying" right now, without emphasizing the gradual progression of the change.

Since this lesson also covers venir + gerund, could we say viene variando here instead?

Yes, but it changes the timeline slightly. Viene variando implies that the pronunciation started varying in the past and has been varying up to the present moment. Va variando focuses on the gradual change moving forward, or simply the ongoing, step-by-step nature of the change.

Does notablemente translate exactly to "noticeably"?

Yes, it works perfectly for "noticeably" as well as "notably" or "significantly." Just like in English where you add "-ly" to an adjective, in Spanish you add -mente to the feminine form of an adjective. Since notable ends in an "e," its form is already the same for both masculine and feminine.

Could I put the subject, la pronunciación, at the end of the sentence?

Absolutely. You could say Va variando notablemente la pronunciación. Spanish word order is quite flexible, and putting the subject at the end is very common when you want to put more focus on the action itself rather than the thing performing it.