Future tense

[sunny1 2]

Is the classification for will and am going to comprehensive?

For example, suppose you have the sentence "You will shortly hear music" or "You'll...", which indicates a certainty.

I have heard this kind of construction used many times. More often than "going to" and often with the contraction.

Is it grammatically incorrect?

[Minoo 233]

No, it isn't incorrect. 'will' and 'going to' are often interchangeable when it is irrelevant whether something is someone's intention or a series of certain events planned in advance.

In presentations, we often do this: Today, I'm going to speak about ...., Firstly, I'll ..., then, I'll ...  etc.