I'm running a new iMac. 2GHz, G5. OS X 10.4.1

I'm having a reaql problem with audio input. I've been on with Mac tech 4 times and have been through all the diagnostics, and have even reinstalled the OS plus reset NVRAM. No luck...I can't get the internal mic or external mic to work at all...when I thump the "Apple" on the base i can get some form of response, so it looks like the speaker isn't dead...but on the sound card there's this situation where it looks like all is mute. Aplle Help seems stumped, but I have several Macs, and none have this mute indicator on. Any thoughts on how to remedy? Here's from the SysProfiler.

Devices:
Burr Brown PCM3052:
Inputs and Outputs:
Internal Microphone:
Controls: Mute, Master
Playthrough: No
PluginID: Onyx
Line Level Input:
Controls: Mute, Master
Playthrough: No
PluginID: Onyx
S/PDIF Digital Output:
Controls: Mute
PluginID: Onyx
Internal Speakers:
Controls: Mute, Left, Right
PluginID: Onyx
Line Level Output:
Controls: Mute, Left, Right
PluginID: Onyx

[ohara]

the sound system on my rev a iMac seems to be the only thing buggy in hardware i've noticed. sometimes the sound cuts off with this awful noise.

what i have the most difficulty with, however, is getting the iMac to recognize audio in lines and such.