The main draw for DS Sound is the new SD card slot. Out with the GBA slot, the SD card slot enables users to put sound clips or music on their SD cards and then pop them into the DSi. A nice addition for some.
The actual DS Sound interface has to be the handheld's biggest let down. Sure, it's possible to record a ten second sound clip and then speed up the clip, slow it down and alter the pitch. The sound effects are surprisingly limited: there's a robot voice, a helium voice, trumpet, whistling, etc.
DS Sound does not feel robust, and honestly, it's a feature I can only see appealing to those who have not grown tired of turning sound clips into Chipmunk music. While the DSi Camera did have me doodling, DSi Sound didn't exactly have me noodling.
Sure, the SD card slot is to future-proof the DSi, but right now, I can't help but thinking I'm better off with a GBA slot.