axwii: re-add the missing sync -- might work better with the new MailHandler implementation

Source/Core/Core/HW/DSPHLE/UCodes/AXWii.cpp

@@ -607,10 +607,7 @@ void AXWiiUCode::OutputSamples(u32 lr_addr, u32 surround_addr, u16 volume, bool
   }
 
   memcpy(HLEMemory_Get_Pointer(lr_addr), buffer, sizeof(buffer));
-
-  // There should be a DSP_SYNC message sent here. However, it looks like not
-  // sending it does not cause any issue, and sending it actually causes some
-  // sounds to go at half speed. I have no idea why.
+  m_mail_handler.PushMail(DSP_SYNC, true);
 }
 
 void AXWiiUCode::OutputWMSamples(u32* addresses)