Using Audio Input of computer as Oscilloscope for Hobby Electronics
Generally speaking, the audio inputs of a PC are terrible.  Lots of noise, terrible frequency response, etc.  

For example, the frequency response of a PC audio input is typically 20Hz to 20 KHz, but could be much worse.  Some audio inputs are only really designed for voice, which has a frequency band of about 300 Hz to 4 KHz.

But none of the PC audio inputs will be good down to DC (0 Hz).  This makes the usefulness of this as an o-scope rather limited.  

This is useful if you are looking at analog signals that are within the 20Hz to 20 KHz range.   It can be very useful, actually, for this.  But when looking at digital signals, or signals that have components outside of this range, then the usefulness quickly vanishes.

Here's the thing:  You want your test gear to be trustworthy, and using a PC audio input is anything but trustworthy.  Yes, there are some situations that you can probably make it work-- but it takes extra work to know if what you are seeing is valid or an artifact of the "scope".   Your mileage may vary.  For many things, it just isn't worth the effort to use the PC audio inputs in that way.