mp3 is lossy compression, it will sound worse than wav/flac, it's always been like that
however, compression settings and mp3 encoder used do make some difference. try 320 kbps and make sure the sample rate is not terribly wrong (44.1 for mp3s seem to be common though even if recording sample rate is higher). it's also worth trying to encode the mp3 with a third-party encoder like lame (-V0 or something like this). if none of those will get you close to what you want, you're probably hearing the limitations of mp3 format and that's it.