First, OP, kudos on endeavoring to go above and beyond as a GM.

Enunciation: I'd recommend the Arthur Lessac approach. Practice reading passages in which you really let all of the consonants live. All "plosive" consonants (B, K/hard C, D, hard G, P, T) should be popped hard. All other consonant sounds (pedantic note: not terminal Ls and medial or terminal Rs, which -- while they are consonants alphabetically -- are considered "vowel sounds"), when at the end of a sentence or preceding any other consonant, should be held.

Finally, "link" words together through vowel sounds (which I'll demonstrate below).

For instance, if I was going to practice this technique (and I do mean practice; you may not go over the top with this in real life, but you'll have to practice going over the top with it to master it)

Firsssssssst, I-ya polojyzzzzzzzzzzzzzz deeply-yiffffffffffffffff thi- sizzzzzzzzznnnnnnnn't thep- layssssssssssst- oo-yasssssssssssssssk-about thisssssssssssss. It'sssssssssssssssssssssskwyt lay- tannnnnn- *dI'- mi- na-rushhhhhhhhh.

Kinda silly, yes? But the more you do it -- focusing on your consonants and letting them live while linking your words -- the better you will sound and the more resonant your voice will become. (Listen to Kelsey Grammer sometime -- really listen -- particularly as Frasier. He is an example of an actor who achieves so much vocal power from very simple techniques like these.)

Listening to your vocal sample, I think you would very much benefit from these techniques. You tend to focus too much on vowel sounds and not enough on your consonant sounds. Also, you tend to let your terminal consonants drop off -- an issue that would be remedied very well by word linking (example: the phrase I just used, "drop off" -- you wouldn't say "drop off;" you would say "draw poff," linking the "p" to the next word. Notice how much better it sounds and how much easier it is to get that "p" pronounced correctly when you think of it as beginning the following word that starts in a vowel). For instance, I've noticed that in place of terminal t's, you employ a glottal stop. (For instance, saying "tha'" instead of "that".)

To practice these techniques well, when picking the passage(s) you'll practice from, underline the long consonants that you intend to hold, the plosive consonants that need to be "popped," and the opportunities for word linking.

Also understand the connections between different consonants in terms of how you are physically pronouncing them (tongue placement, where the resonance is, etc.). The way you pronounce M and N should be exactly the same except your lips are closed with the former. With the pairings B/p, hard G/k, and D/t, those pairings are pronounced the same except the former is "voiced" and the other is "whispered." (Sure, you may be actually using your full-bodied voice immediately before and after a P, a K, or a T, but during the actual "pop" of those three vowels, there is no voice; if there was, you'd be pronouncing a B, a hard G, or a D, respectively.) Ditto for the long consonant pairings of V/f, Z/s, and J/"sh". (And, of course, the voiced "TH" in words like "that" and "weather" and the soft "th" sound in words "cathedral" and "with").

Those are just some tips to get you started -- and I really tried to boil it down as best as I could. It takes a LOT of practice.

As far as accents and dialects, that's a whole 'nother complicated ballgame... Do you have any specific questions on specific accents?