I want to seal up the drain holes, (I guess they're drain holes), in the bottom of the doors with dynamat to kill some road noise and make my tunes sound better.
(If you've looked into the speaker hole with a flashlight, at the bottom of the door, you know what I mean, you can also see them if you were laying on the ground and looking up at the underside of the opened front doors.)

I've never seen any water or moisture in there anyway in the times I've been into the door interior.

Will this deaden some road noise or am I asking for the bottom of my door to rust out?
(Don't have any rust problems in Houston anyway, no snow or salt)

And has anyone dynamatted their doors, how much of a difference did it make?

 

If you have no rust in the doors now you will if you seal up the drain holes. You have to remember you will get water in the doors when it rain and will have no way for it to come out.

 

x2

 

Those drain holes are why you don't see water in there. Rain sneaks down the side glass right into the door, the holes are there to let it back out.
You really won't gain anything much by applying dynomat to the door bottoms anyway. Sides, sure.

 

Dynamat it is then, don't know how you stuff it thru that little speaker hole but I'm going to try.

Yes I want it on the interior of the outside of the door.