You make the /m/ sound a bit longer. First you make a sound like you are humming for the first m and then you read the second m normal. But these are said together without pausing. I hope that helps.
I wish Duolingo supports the ability to add voice notes. I was going to add a voice note on how it is read. Write to me on my wall. I may find a way to read it to you.
Is there no pronunciation recording for the Swahili module or is my device defective?