Maybe it is because the Converter you use can't convert 5.1 surround sound. Just try another one to convert MKV to MP4.
If you have no idea, you may prefer to this step by step guide on how to convert MKV to MP4. This guide can handle all situations like MKV to MP4 with subtitles, MKV to MKV with 5.1 surround sound etc.
Maybe it is because the Converter you use can't convert 5.1 surround sound. Just try another one to convert MKV to MP4.
If you have no idea, you may prefer to this step by step guide on how to convert MKV to MP4. This guide can handle all situations like MKV to MP4 with subtitles, MKV to MKV with 5.1 surround sound etc.