You can now add a low pass fade to your song whereby as the fade out goes through, the higher frequencies are gradually attenuated creating a sense of the sound moving further away.
Just pull the fade back from the top right of a media item – add your fade, use option / alt modifier to change the shape.
If you right click the fade curve line and you have Reaper 7.48 you should now see the option to add ‘Low Pass Fade’ to your fade out.