You can also see our wiki which has many related definitions and tips; while PC-ALE is the weapon of choice for many, there are other packages that might work better. Every soundcard is different and it's nearly impossible to quantify them in this regard...
ALE - The RadioReference Wiki
ALE is not just used by hams; there's a HUGE number of government and non-government users of this mode. There are literally hundreds of logs of this mode in the UDXF every month.
Use a source with a fixed audio level. Too much audio and you won't get any decodes. Hopefully your PC has a line in; check your system tray as it's not at all uncommon for this to be disabled coming from the distributor. The line in is your best bet to feed your PC. A
short connection using shielded audio cable (to stop the possibility of any noise from other sources getting into the audio signal) is a real good idea.
Mike