Here is a guide for using custom music commands
This guide is step by step and is super simple and easy to do

--By Pressing your Console key Normally [`] on your KeyBoard you will open the console--

--There are 2 commands to run here, First slender_customchasetheme then slender_customsearchtheme--

--Type slender_customchasetheme in your console, First tho we need a sound to put in here--

--We need to make a folder in the addons folder, So here is what we do--

--Goto your steam library, Then goto garry's mod and right click on it, You should now see properties and click on that--

--Now look for installed files and click on that--

--Now click browse--

--Open the folder garrysmod then look for the folder named addons--

--NOW in here is where your gonna put your sounds--

--But first we need to make a new folder in the addons folder--

--For example for this guide we make a folder named slender_custom--

--In slender_custom folder make another folder named sound--

--Then i make another folder named custom_music--

--Now put your sounds in here-- Note Wavs work the best so id use that

--Now for example we have this sound slender_custom_chase.wav and slender_custom_search.wav--

--Now again, Go back into your garry's mod application and type in the sound path custom_music/slender_custom_chase.wav for the chase theme--

--Then for the search theme type slender_customsearchtheme custom_music/slender_custom_search.wav--

--Now it should end up like this in your console
slender_customsearchtheme custom_music/slender_custom_search.wav
slender_customchasetheme custom_music/slender_custom_chase.wav
--

Now restart by loading into slender mod/Rejoining the server your in and It should now be loadad