To fix the custom sounds, we need a software that edit sounds.
Any software would work. but we will be using the following program for this tutorial.

Audacity[audacity.sourceforge.net]

Install it, open Audacity and it should look like this.

• File > Import > Audio
  
• Edit > Select > All
  
• Edit > Copy
  
• File > New
  
• Tracks > Add New > Stereo Tracks
  
• Edit > Paste
  
• File > Export as .wav

Import the sounds you want to use in Source Filmmaker, by clicking File > Import > Audio
And select the sounds in the file explorer, click OK, and a progress bar will appear for a few seconds.
Once it has finished importing, the waveforms will appear in Audacity like this.

The reason why your sounds is inaudible in Source Filmmaker, is that your sounds aren't meeting the Source Filmmaker requirements, which is 44100kHz 16 bit .wav files. (I know .mp3 is acceptable, but we will be doing the .wav, the result will be much more accurate)

Now take a look at the bar on the bottom of the screen in Audacity.
The Project Rate (Hz) says 48000, which has exceeded the requirement to import an audible sound.

Now that we know what is happening to your custom sounds, let's start fixing the sounds.
Click the Selection tool, and click Edit > Select > All (or Ctrl+A) to select the entire waveform.
Alternatively, highlight the waveform by clicking and dragging across the waveform.



After the sound is highlighted, click:

• Edit > Copy (or Ctrl+C)
  
• File > New
  
• Tracks > Add New > Stereo Tracks (This is where you make your sounds audible to Source Filmmaker)
  
• Edit > Paste (or Ctrl+V)
  
• Wait for a few seconds and the waveform will appear.

Look at that, it is 44100 ! We finally met the requirement for the Source Filmmaker to accept custom sounds ! So click File > Export, and export it as "WAV (Microsoft) signed 16 bit PCM"

Now your sound should work in Source Filmmaker.
Place your sounds that you want into Steam\steamapps\common\SourceFilmmaker\game\usermod\sound
Open up Source Filmmaker, add Clip to Track, load up the sound you want.
Click Rescan if you already have the Source Filmmaker opened up and you can't see your sound file.

If you can hear the sounds, congratulations !
You now have a working custom sounds in Source Filmmaker.

Hope this helps ! Comments or feedback is also greatly appreciated !
Rate this up if this really help you, it helps the community a lot.