If I amplify some audio file (maybe because it's unhearable before) using some amplifying tool, how can I know that playing the new (output) audio file won't damage my speakers?
No matter what the maximum volume is of the file, your amplifier will have input and output maximum. If the input is too high, your gain stage will clip - generally not pleasant, but it limits the signal. And your speakers should be powerful enough to cope with the maximum your amp can push out. So if you have a 100W amp, having speakers that can cope with 150W is likely to be just fine. I have often used 150W amplifiers with speaker stacks that can cope with 250W. That way I know the limiting factor will be my ears, not the speakers.
More generally, just play the first time with low gain/volume... something like this is how bands will set up in any new venue with unknown sound desk etc:
- Have all gain and volume set to zero
- Turn on kit
- Start playing
- Increase input gain until signal is at correct level for amplifier
- Increase volume to get desired output
- Monitor in case changes are needed