From what I can tell, the problem is with the alignment of data in the header which is read using a struct rather than field at a time. This patch uses stdint to ensure the correct sizes for all of the fields. Duplicate of Bug 95966 (which is marked "invalid")
Created attachment 259110 [details, diff] Patch for amd64
Created attachment 259111 [details] Updated ebuild to apply the patch.
Fixed in wavplay-2.0 in CVS.