Dripping Sound from inside toilet tank

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.

If it occurs during summer, and days when you run your air conditioner, its likely they fed the condensation drain pan to the house drain. Try turning off your AC for a whole day and see if you ever hear it.

Its also possible that its someone else's AC unit that is drained to your drain, or in a multi-unit building, the common drain that happens to run through your wall.