I did the same thing too except I created a sub-folder within the roms directory and called it mame-samples and edited both the 1.4x and the 0.9x versions of the mame-advmame.rc file and referenced my folder there as I found some of my games were using the 0.94 version while others were using the 1.4 version of the rc file.
On the sample path line, I appended the updated folder path to the existing entry.
dir_sample /home/pi/RetroPie/roms/mame-advmame/samples:/home/pi/RetroPie/roms/arcade/sample:/home/pi/RetroPie/roms/mame-samples
Any new audio samples I'll download will go to this folder instead of within the samples folder of ADV-MAME.
The samples work when they're zipped, not extracted.