A lossy MP3 is encoded at 192 kbps by default. The Audio Quality setting also offers 320 kbps for the best quality and 128 kbps for a smaller file. A lossless MP3 (WAV, FLAC) ignores the bitrate and keeps every sample.
Usually the MKV carries more than one audio track and the one that was converted is silent or secondary. The converter picks one audio track automatically, normally the main one, and this page has no setting to choose a different track. If the wrong one comes out, remux the MKV with the track you want first (VLC or ffmpeg can do this) and convert it again.
