Map volume keys from 0 to >100

[tkn]

Hi potac, welcome to Ubuntu-MATE community.

First to answer your question:

I cannot manage to map my volume keys to go above 100%.
Any idea if this can be achieved in mate?

Not that I know of, it may be possible with a hack but it is also not really the best way to solve the problem.
Boosting the output-stage beyond 100% is a useful (and quick) hack in some cases but (technically) not a good solution.

see:

Am I the only one around here who hates this dual volume stuff?

The Control Center in 21.10 no longer has an icon for Sound. You can find a setting in Ayatana Indicators for Sound that let's you choose "Allow setting the volume above 100%." Is that what you are looking for?

If you want to boost the ouput volume beyond 100% because your input volume is too low, you're actually trying to solve the problem on the wrong end of the audio-chain.

Your audiosource should give you the tools to boost the levels to nominal volume ( 0dB peak ).
I don't know what your audiosource is but if it is a mediaplayer like VLC you can boost it to 200% (with , of course, the possibility of loads of distortion if your peaklevel gets above -6dB)

EDIT: If your audiosource doesn't give you the possibility to change volume above 100% or change volume at all you can install PulseAudio Volume Control (see answer of @snow_dog ) and use the playback sliders to fix this.

If you want to boost the ouput volume beyond 100% because your output volume is too low while your input volume is OK, your output amplifier is definitely not up to the job and will go into clipping like mad. I wonder if it will survive that for long.

If you want it louder without giving up dynamic range and without loads of distortion you need more headroom:

1. If you are listenin on PC-speakers you might turn the volume up on your PC-speakers.

2. If you are listening build-in laptop speakers, install pulseeffects.
   sudo apt install pulseeffects
   because almost all laptop speakers are so incredibly bad you probably need more than just boost the volume. You will probably need a parametric EQ and a compressor to just make it bearable.

3. if you are listening on a laptop through headphones you might want to use a seperate headphone-amplifier because there are a lot of laptops out there that have incredibly inadequate output-amps that are, while OK to use with simple earplugs, not nearly-enough to drive a good quality headset.

tl;dr: If you don't care about any of this and only want a quick solution,
install pulseeffects
sudo apt install pulseeffects

tl;dr: If you want the right solution, install PulseAudio Volume Control.
sudo apt install pavucontrol

EDIT: thanks @snow_dog

Good luck and happy hacking