effect of the muffled music

[hagiel120]

Do you know how to do the music at some point was muffled, like from behind the wall?

[kozikowski]

Try Effect > Equalization and make it look like the attached.
Koz

[steve]

or the Low Pass filter: http://manual.audacityteam.org/o/man/low_pass_filter.html

[Trebor]

It can be progressively-muffled with this variation of Steve’s code in the Nyquist Prompt …

        (setq start-freq 22000) ; cannot be greater than 1/2 the sample rate
        (setq end-freq 200)
        (setq passes 6)         ; more passes for a steeper filter cut-off
        (setq sweep-type 0)     ; 1 for a linear sweep, 0 for exponential sweep

        (let* ((nyq (/ *sound-srate* 2.0))
               (f0 (max 0 (min nyq start-freq)))
               (f1 (max 0 (min nyq end-freq))))
          (if (= sweep-type 0)
              (setf lpfreq (pwev f0 1 f1))
              (setf lpfreq (pwlv f0 1 f1)))
          (dotimes (i passes s)
            (setf s (lp s lpfreq))))

https://forum.audacityteam.org/t/nyquist-code-for-gradual-high-pass-filter-please/30601/1

If you reverse the track then apply the code , (then reverse it back) , it gradually becomes less muffled,
similar to … https://soundcloud.com/glitchhop/amb-get-loose#t=2:33