How to remove this background noise

There's probably not a lot you can do since (if you think about it) the music and other people talking will occupy a similar frequency band to your mum's voice.

Without hearing it, I can only guess but you might be able to get more intelligibility by EQing your track to boost your mother's voice and slightly cut everything else--to determine what frequencies to use, you'll have to experiment and use your ears.

Also, you can sometimes do some surprising things in Waveform view with the Spectral Frequency Display.  Basically you draw around some of the frequencies you want less of and delete them.  Again you can only judge this by ear.

If you'd like some of us to have a listen to your recording and try to be more specific, you could post a short sample on whatever cloud sharing site you like--alas you can't post audio files direct to this forum.

However, in the longer term, the only way to get a better recording is to change things a source and get a mic closer to the person talking--either on a boom/fish pole just out of sight or else a clip on lavalier mic.  On camera mics are next to useless for anything other than a background atmosphere.