Check power on U402 & U403 - U402 pin 4/U403 pin 8 should be +12V, U402 pin 11/U403 pin4 -12V.
Not sure about the exact voltages, but should be >= +-9V - and must be symmetric.
If the power is OK, you can trace the audio signal originating from Paula with a headphone: connect the sleeve to ground with a crocodile clip and touch the tip to the contact you want to check.
PS: hadn't read your post properly - seems you've got a short to -12V. If C405-408 are AOK, check R403 - may be hot (a short somewhere else) or high (doesn't feed the +12V). The schematics don't tell the value, but it should be the same as R404 (I guess ~47Ω). Oh yes, the voltage differential over R403 shouldn't be much (<2-3V).
