I understand that Watson Speech To Text is somewhat calibrated for colloquial conversation and for 1 or 2 speakers. I also know that it can deal with FLAC better than WAV and OGG.
I would like to know how could I improve the algorithm recognition, acoustically speaking.
I mean, does increasing volume help? Maybe using some compression filter? Noise reduction?
What kind of pre processing could help for this service?
the best way to improve the accuracy of the base models (which are very accurate but also very general) is by using the Watson STT customization service: https://www.ibm.com/watson/developercloud/doc/speech-to-text/custom.html. That will let you create a custom model tailored to the specifics of your domain. If your domain is not very well matched to those captured by the base model then you can expect a great boost in recognition accuracy.
Regearding your comment " I also know that it can deal with FLAC better than WAV and OGG", that is not really the case. The Watson STT service offers full support for flac, wav, ogg and other formats (please see this section of the documentation: https://www.ibm.com/watson/developercloud/doc/speech-to-text/input.html#formats).