They sound fairly decent too, I figured out the reed mechanism by looking through the packaging on grunt calls at the store. When you blow into it and the reed closes up it has to be an airtight seal in order for it to open again...if there's a leak, the leaking air keeps the reed sucked closed so you don't get any sound. If the seal is airtight, there is no air movement to keep the reed closed so it opens again but then closes again because you're still blowing...if this makes any sense at all, it's the rapid open/close movement that makes the sound. At least from what I have observed. I use a piece of bamboo about 1/2" thick and whittle the tapered opening on one end and then cut some random scrap of plastic for the reed and glue it in place.

Maybe I'll get a video or recording of it online sometime to see if I can get opinions from experienced hunters on how good they sound. I'll be using one on my first deer trip this coming Tuesday.

If you try to make your own just model it after the ones you buy, and be sure to find a flexible piece of plastic for the reed and make sure it seals airtight when you close it up. I love making things so I came up with my own grunt calls and also turkey tube calls (instructions for those are online though, unlike grunt calls)

Anyway it's fun to mess around and see what you can make. I've tried a couple times to do a crow call but failed because I didn't whittle the bamboo very evenly. I haven't yet taken a look inside a squirrel call but probably sometime I'll try and figure that out too.
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A good day in the woods is best finished off with an ice cold Sun Drop.