I assume the sound output of your tv card goes through line in. Try KMix to deselect line in.
I love the new Knoppix DVD but have a problem I haven't encountered with any other distro of Linux - previous Knoppix included:
I have an old ATI TV Card in my computer and when my machine boots with Knoppix, it "senses" the card and immediate begins playing TV sound. If I try to load the TV software xawtv, the software works flawlessly - as good as the MS software that was made for the card. However, if I close the program, the sound stays on until I turn off the machine.
Of course I could mute it, but finding out how to fix the problem is so much more fun.
I assume the sound output of your tv card goes through line in. Try KMix to deselect line in.

Fujinet 1.7.1 Atari 400 800 800XL 130XE 65XE XEGS Assembled, Tested, Working
$99.99
Atari 130 XE/Atari 1050 Bundle W/ Cords And Extras (Powered On)
$300.00
DEFECTIVE Atari 810 Vintage Computer Disk Drive powers on Unit Only Boo G3D
$49.29
Atari 800 8-bit Computer System Fully Tested Great Condition 48k READ
$350.00
INFO/soft Character Generator Manual for Atari 3000/5000/7000 NTSC
$40.00
Atari XE 130XE Computer NTSC with Power Supply Powers On Untested
$159.95
Atari SX212 Modem for IBM Systems w/ Box, Owner's Manual & Power Adapter NOS
$125.00
ATARI Mouse with Adapter for ST, STE, Mega STE, TT Optical - USA Seller
$29.99
UNOCART-2600 SD card Multi Rom Cartridge for ATARI 2600 with CASE
$34.25
Vintage Atari SF354 Floppy Disk Drive & Power Supply FOR PARTS OR REPAIR AS IS
$120.00