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Every time I boot up I have to switch my audio output device to my "Line Out" headphones because for some reason my microphone defaults as a speaker.

See this screenshot.

Here is a well explained tutorial to set a default audio input/output: Link

First: List the audio output devices using

pactl list short sinks  

Example of the output:

pactl list short sources

0   alsa_output.pci-0000_02_00.1.hdmi-stereo.monitor    module-alsa-card.c  s16le 2ch 44100Hz   SUSPENDED

1   alsa_input.usb-AVerMedia_Technologies__Inc._Live_Gamer_Portable_2_5202050100060-03.analog-stereo    module-alsa-card.c  

2   alsa_output.usb-Blue_Microphones_Yeti_Stereo_Microphone_REV8-00.analog-stereo.monitor   module-alsa-card.c  s16le 2ch 44100Hz   SUSPENDED

3   alsa_input.usb-Blue_Microphones_Yeti_Stereo_Microphone_REV8-00.analog-stereo    module-alsa-card.c  s16le 2ch 44100Hz   RUNNING

Second: To set a default output device run the command

set-default-sink <'output device name'>

Example: pactl set-default-sink 'alsa_output.pci-0000_00_1f.3.analog-stereo'

Now, to make this work at every restart, follow this :

First, open the file /etc/pulse/default.pa using :

sudo gedit /etc/pulse/default.pa

Then scroll to the bottom of the file, where two lines starting with set- will be commented out.
Now, uncomment these lines and replace the words input and output with the number of the sink (for output) / source (for input) that you want to be the default.

Example:

### Make some devices default

set-default-sink 3

set-default-source 3

After doing this, save and exit. Then, delete the ~/.config/pulse directory by running sudo rm -r ~/.config/pulse, and then reboot the system. Once the system reboots, the appropriate devices should now be set as the defaults.

This can be done with PulseAudio Volume Control if you prefer to use a GUI.

Once you've opened PulseAudio Volume Control, choose the configuration (the last tab), then select the output and/or input you want to use from the profile dropdown menu.
It should look similar to this

PulseAudio Volume Control can also be used to adjust volume levels for each input, output, system sounds and applications that are currently in use.

Here's a post from Linuxhint with more info on PulseAudio.