Fix samples signs on win32

examples/music.rs

@@ -33,7 +33,7 @@ fn main() {
                     };
 
                     if let Some(output) = buffer.next() {
-                        *output = next_sample as u16;
+                        *output = next_sample;
                         data = &data[1..];
                     } else {
                         break;

src/wasapi/mod.rs

@@ -40,7 +40,7 @@ impl Voice {
 
     pub fn get_samples_format(&self) -> ::SampleFormat {
         match self.bits_per_sample {
-            16 => ::SampleFormat::U16,
+            16 => ::SampleFormat::I16,
             _ => unimplemented!(),
         }
     }