Noise in audio.

We are building a mini laptop by assembling a Single board computer (SBC), DC-DC converter (24V i/p 5V and 12 V o/p ), LCD, mouse, 4x4 matrix keyboard etc.. for a particular application.
We have one more module which is connected to SBC through PC104 and line-in.
We have designed a PCB which holds this module and SBC together. It is used for routing lines of pc104, audio, mouse, usb, vga, ethernet etc. It is multi layered and one layer is ground (reference line) itself.
We have used 2W audio amplifier ST TDA7268 which takes i/p from line-out and feeds to a 8 ohm speaker.
computer is loaded with Windows XP.
All the modules work fine when tested seperately..
Problem: When the computer is on there is a constant humming noise (loud enough) in the speaker. Also while moving the mouse (no matter usb or ps2) some other noise adds to it. Same is the case if you keep pressing a key in the keyboard.
If you mute line-in (in windows) there will be no noise. (but we can't do that as we are connecting our module to line in). Can't just blame on that module because this module is working fine in some other application without any noise.
Noise persists u completely dettach the module (dettach its power in, dettach from line in and pc104 of SBC).No matter line in is on or off (in windows).
No noise if you connect the module to only power and pc104 (dettach line-in connection). Once Line-in is connected(on board) and make line-in on(windows), noise appears.

I think problem is with grounding. We are using 24V DC battery as power supply and there is no earthing.

Please help me how to find the source of noise... Also if it is problem with noise how to solve it. (we cant change pcb design)