The low volume sound you hear from your siren is a bad ground on the brain using the siren ground for the alarm. This would also be the problem you are also having with the lights and door locks.

Since your alarm is very old, Viper offers an upgrade program. This offers you a brand new alarm at a discounted price, but the catch is you have to be the original owner of the alarm. Find a reputable authorized Viper dealer and check with them about it.