Are using and IDE 1.0.1 or later?
Change these to
and wire your switches to connect the pin to Gnd when the switch is closed.
Change your logic to match:
This will ensure that the HIGH/LOW level is always valid, and not "floating" as the pins are read.
You don't do anything with switch 5 yet, I assume more will be added later.
I am kind of surprised any thing plays. As soon as you reach an if condition without a key pressed you stop the tone. You may want to re-think your logic there.
Perhaps only call noTone(skp) when a key had been pressed and is then detected as not pressed; that way, key2 does not stop the tone from key1.