> My last PC had some connections on the motherboard to bridge the sound
Hard to say; each PC, even the same brand, can have different setups and
you didn't mention anything useful to answer about. However, although
it might work to connect it to the Line In, based on your apparent level
of possible expertise it may be inadvisable due to the ease with which
static discharge, even those you cannot feel, could damage your system.
Doesn't your TV tuner have instructions on how to handle the audio?
Check the docs. What you're trying to wire should already be possible
with electronic switching on the mobo. Or the tuner card may have an
audio output? Or one of the connectors on it does. Or it's already
available at one of hte sound card outputs.
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