core: `NullAudioBackend` should not report latency seek data as part of the size of a sound.

core/src/avm2/globals/flash/media/sound.rs

@@ -64,9 +64,7 @@ pub fn bytes_total<'gc>(
 ) -> Result<Value<'gc>, Error> {
     if let Some(sound) = this.and_then(|this| this.as_sound()) {
         if let Some(length) = activation.context.audio.get_sound_size(sound) {
-            //TODO: The length of the sound data is consistently off by two in SWFs.
-            //This probably needs to go away when we start decoding network sounds
-            return Ok((length - 2).into());
+            return Ok((length).into());
         }
     }
 

core/src/backend/audio.rs

@@ -135,6 +135,13 @@ impl AudioBackend for NullAudioBackend {
     fn play(&mut self) {}
     fn pause(&mut self) {}
     fn register_sound(&mut self, sound: &swf::Sound) -> Result<SoundHandle, Error> {
+        // Slice off latency seek for MP3 data.
+        let data = if sound.format.compression == swf::AudioCompression::Mp3 {
+            &sound.data[2..]
+        } else {
+            sound.data
+        };
+
         // AS duration does not subtract `skip_sample_frames`.
         let num_sample_frames: f64 = sound.num_samples.into();
         let sample_rate: f64 = sound.format.sample_rate.into();
@@ -142,7 +149,7 @@ impl AudioBackend for NullAudioBackend {
 
         Ok(self.sounds.insert(NullSound {
             duration: ms as u32,
-            size: sound.data.len() as u32,
+            size: data.len() as u32,
         }))
     }