Reading analog sensor as a microphone signal (via sound card)
The sound input on a PC card (mic, line etc..) is AC coupled and this means it won't help if the signal you are trying to read is constant - there is a capacitor which blocks DC and prevents this. However, for a piezo sensor, its output is AC so this shouldn't matter except that at low frequencies the sensitivity of a sound card gets progressively less and less (due to the capacitor dc block circuit) below frequencies of about 20 Hz.

As for capturing signals, any wave analysis software will do. I use wavelab for this sort of thing and it's got a decent suite of filters and signal generators. Google "free wave edit software" - there are plenty of options.