Bitwig Studio is officially released for Ubuntu, which uses Libav (a fork of FFmpeg), while many other distros use FFmpeg, that impedes from importing mp3, FLAC and other codecs files. Instead of installing Libav, which may not be in repositories and may require to remove FFmpeg, there is a simple workaround:

sudo ln -s /usr/bin/ffprobe /usr/bin/avprobe

You may find more informations in this KVRaudio forum thread.