I think that’s an alto, Kenneth. But, in any case, unless I could return it, I’d open it up and take a look inside. If the reeds are discolored, it’s not new. The HM-901 is not being marketed anymore. It sounds like there may be something interfering with some of the reeds. Maybe some debris got in there — food particles, lint from cloth used to clean the reeds, or whatever. I found a dog hair in one of my melodicas. You’d have to ask my dogs how it got there. My guess would be that the strange sounds are caused by something that would be visible. Or possibly some of the reeds have been damaged by someone blowing too hard or trying to bend notes too far. I have read on harmonica sites that these kinds of reeds can develop very fine cracks. I don’t have a button type of melodica, but my experience with other melodicas is that they’re easy to take apart and reassemble and it’s not something you should be afraid to do. Just use a good screwdriver of the right size. Take note of which key isn’t playing at all, and examine the reed of that key once you have it apart. A possible issue there is gapping. You might find that the gap is noticeably smaller or larger than the gapping of its neighbors. It’s very easy to break a reed off, as they’re more brittle than you might expect. But if the gap is too small, i.e. the reed is flattened down, you can (at your own risk) slide a business card under it and very gently widen the gap, being careful not to go too far.
But I’d wait first to see what other responses you get from other members who have worked on this type of melodica.