Correct, with seremala the m- denoting a person is not used. It is a ma- class noun and not a usual m/wa class noun. Therefore the plural is maseremala, not waseremala.
The audio sounds like seremara. It's not really wrong as you will encounter this depending on people's language background. Most languages in East Africa don't distinguish between /r/ and /l/ and among native speakers' dialects of Swahili there is also considerable variation. Seremala is the standard spelling and pronunciation though.
Is it because 'seremala' refers to a person that we use 'mzuri'--an m-class word?
Yes, even though seremala is a ma- class noun, since it refers to a person it takes m- class agreements.