another z24 that won't start...this one's different though

1992 z24 3.1 wouldn't start...replaced starter motor/solenoid & battery and it worked fine for about a month. all of a sudden i try to start it one morning and there's nothing. multimeter says battery is at 9 volts...how can this be? didn't drive it for about a day and a half prior to this. my voltage was fine before now...12.5 to 13.5 volts. tried to jump...it cranked a little more than w/o cables, but no start. i turn the key and there's no action or sound. anyone know what's up?