From what you have said, it sounds like a better way to go might be to replace that top edge of the brass outlet pipe. They sell kits in the store to replace the flapper valve and the seat (which is what you are struggling with). Have you looked into one of those? Something like this:
http://www.amazon.com/Lavelle-2003BP-Flush-Valve-Repair/dp/B001ARJR8S
Trying to sand the existing seat down, in place, until you get a good seal sounds like a lot of work with little chance of success. With a kit like this, you seal a new plastic seat down on top of the old one with a silicone sealant, then the flapper seals against the new seat. If one of those seat replacement kits will adapt to your toilet, that is your best bet, in my opinion.
Hope this helps.
This is a very controversial topic. Some plumbers swear that you must seal the toilet to the floor, while others swear that you should not seal the toilet to the floor. Some guys never do it, some guys always do it, and some guys only do it depending on the flooring used. It also appears that some toilet manufacturers mention it in the installation instructions, and some do not.
I've heard it mentioned that "it's code" to seal the toilet to the floor, but I've never heard anybody cite the actual code that says it. I'm not well versed in plumbing code, but a quick look didn't reveal any obvious code sections. Though plumbing code (and most other codes), does require you to follow the manufacturer's installation instructions. So if the installation instructions say to do it, then you should do it (if you want a code compliant install).
In my opinion, it's a double edged sword. On the one hand, sealing the toilet can keep water and grime from going under the toilet. At the same time, it also keeps water from coming out from under the toilet. If you develop a leak, the water will be trapped under the toilet. This could allow the leak to do much more damage, before you notice you have a leak.
If you flush the toilet and get wet feet, you're going to call a plumber to check the toilet. If after years of flushing the toilet you notice a strange smell, and maybe a water stain on a ceiling somewhere. You're going to start ripping things apart searching for the source of the leak.
To try and alleviate this negative side effect, some plumbers leave a gap in the caulk near the back of the toilet. While this may let water from a leak out, it also allows water in. Which defeats the purpose of sealing the toilet to the floor in the first place.
If you're installing a toilet in a bathroom where kids are going to be bathing (splashing), it might be worth it to seal the toilet. If you're installing a toilet in a water closet, where there's not likely to be water near the base of the toilet, then maybe it's not worth it. Follow the manufacturer's installation instructions, and use your own judgment.
After doing a bit more research, it does appear that codes require the toilet to be sealed.
International Plumbing Code 2012
Chapter 4 Fixtures, Faucets and Fixture Fittings
Section 405 Installation of Fixtures
405.5 Water-tight joints. Joints formed where fixtures come in contact with walls or floors shall be sealed.
Uniform Plumbing Code 2009
Chapter 4 Plumbing Fixtures and Fixture Fittings
Section 407 Installation
407.2 Joints. Where a fixture comes in contact with the wall or floor, the joint between the fixture and the wall or floor shall be made water-tight.
NOTE: This section may have been moved to 402.2 in 2012 UPC. I did not have access to a copy of the 2012 version to verify.
Best Answer
If the sound is only heard when material is added to the bowl, you can disregard any involvement of the flushing mechanism. This must logically be the case, unless you omitted to mention that you also flushed the toilet afterwards, and/or that the liquid had entered the bowl from the water tank. If the latter is the case, the tank would make a gushing noise while it was refilling itself, and possibly some dripping sounds for a little while afterwards while the flushing mechanism settles down.
If the sound occurs when material is added to the bowl, but no associated flushing takes place, the only possible source of the dripping sound is the overflow from the S-bend when the liquid displaced from the bowl falls or runs down the sewer pipe. If there is no leakage from the toilet when you hear the sound, you can safely disregard it.