Apparently "beat songs" do not have constant tempos throughout the song... For example, they may vary between 125 to 130 beats per minute. Is there a way to make them "beat" at a constant and exact rate (e.g. exactly at 130 throughout the whole duration of the song)? Second, is there an open-source way to do this?
According to https://superuser.com/questions/129041/any-beat-detection-software-for-linux, some time ago there was the BpmDj project, but now it seems abandoned and I cannot get it to work. It seems unrelated to what I want to do, but it could help me.
Edit (after being enlightened by Warrior Bob)
It seems audio technology has evolved a lot last years. Ableton defines this "beat-matching with metronome" (or time streching) as "warping". I tried the "non-beat" "boxer of Simon & Garfunkel" with Ableton. I was amazed of how accurately it finds the beats/tempo. I "warped" it in about 1 hour reading one simple manual page and watching some demonstrating videos. I suppose an experienced user could do it in even (quite) less than 10 minutes. Last, this is also possible with BitWig Studio and it runs natively in Linux so I'll probably stick to BitWig!