resolved - look for mame4all_1.0_samples.zip and, for original space invaders... also invaders.zip (sound files) and ftp up to /home/pi/pimame/emulators/mame4all-pi/samples
-suggestion: could the web based upload tool recognise sound zips and put them in the right place?
problem 3 - intermittent freeze of joystick/keys
resolved - USB power issue.
using the picade usb board (which appears to a keyboard to the rPi, all was fine until actually loading up a game. after about 20 seconds the controls would freeze and some faultfinding with lsusb and evtest it appear that the whole device would disappear and reconnect. lots of head scratching later maybe speakers are using too much of the available power.
I have one of the very first rPi's and figured maybe USB drivers out of date.
sudo apt-get update sudo apt-get upgrade
then to update the drivers sudo rpi-update
and issue is gone, sound is quieter (and a more normal level!) and no more freezing.
putting a powered USB hub between the pi and the picade might also solve this but mine is still directly connected.