is it possible somehow ?

I need it to move several audio keyframes.

For example, if you adjusted the music with auto gain / keyframes and you have to cut out something in the beginning, you have to move every single keyframe which is not very effective.

why can't I select in and out, press alt and move all keyframes horizontally ?

Unfortunately your not able to move a group of keyframes horizontally, only vertically.  Not sure if there's a huge need for this feature but you could post it in the feature request forum regardless if you'd like.

I think it would be useful, especially when playing around with timewarps.

Sorry are you using Media Composer or Xpress Pro?