a NO NO!
synths are normally line out level. you should connect the synth line out to the sound card line in. NEVER the mic in.
actually be also aware its not a good idea to plug and unplug to sound cards
while the pc is booted up ANYWAY. you can jolt the circuits in the sound card. ive seen some go out of action this way.
this is why when i record a synth i plug it into a mixer , and then go line out
of the the mixer into sound card line input.
if you provide more detail what your trying to do i'll try to help.
peace.