Hyphens – Is ‘Forty-Five Year-Old’ Correct?

Your examples aren't exactly the same. In your first example, you're actually missing a comma:

I heard his deep, warm voice filling the room.

You need the comma here because both deep and warm describe voice. What kind of voice is it? A voice that is deep and warm.

She was wearing a deep crimson shirt.

No comma here, and also no hyphen; deep describes crimson, and crimson describes shirt. What kind of shirt is it? A crimson shirt. What kind of crimson is it? A deep crimson.

I think '32-bit' being hyphenated is just because it's become a convention to write it that way. If you wrote 32 out as a word, you wouldn't hyphenate:

The thirty-two bit Pentium had a sixty-four bit data bus.

ETA: Upon further research, it seems that in the first example, you can either use the comma or not use the comma; either would be correct. (Source)

I found this example online that illustrates when to use the hyphens. Basically they are used to avoid syntactic ambiguity; when not using the hyphen makes the sentence mean something else, you use the hypen.

• Jacob took the well-fatted calf to the riverside. ('well-fatted calf' as in a very plump calf)

• Jacob took the well fatted calf to the riverside. ('well fatted calf' could be construed as a 'well' (i.e., healthy) and 'fatted' calf. In the first example, the 'well-fatted calf' could be ill.)

This is similar to your added example about the well-known actress:

If you do not use a hyphen, you have:

The well known actress accepted her award.

In this case both well and known apply to the actress. This means she is well (not ill) and known. But that's not what you want to say; you want to describe to what degree she is known, and you don't mean to say anything about the state of her health. So you use the hyphen to make well-known into a compound adjective everyone will understand:

The well-known actress accepted her award.

In your deep crimson shirt example you don't have any ambiguity. A shirt cannot be deep, so no one is going to be confused if you don't use the hyphen. We understand inherently that deep applies to crimson and crimson to shirt. So it seems we use the hyphen to make compound adjectives when otherwise the sentence's meaning would change.

As per StoneyB's comment, the hyphen is effectively optional.

In OP's specific example, most people probably would include it, but grammatically the usage is no different in, say,...

a recently published review

...which as you'll see if you follow the link, most writers don't hyphenate.

Since language is primarily spoken, one must be careful when suggesting the presence or absence of the hyphen has any semantic significance. But sometimes there really is a difference - for example,...

"It won't work properly unless you do the right mouse click"

...where it's possible the right (correct) mouse-click is in fact a left (or middle) click. This is one of those rare cases where (in principle) the written form can be less ambiguous than the spoken form...

1: right-mouse-click (definitely the one on the opposite side to the left-mouse-click).
2: right mouse click (completely ambiguous)
3: right mouse-click (potentially ambiguous)

Some native speakers will think they can unambiguously indicate whether they mean right=correct or right=[opposite of left] depending on stress patterns and pauses, but in practice many native listeners still won't be 100% sure what was meant.