Wav files are distorted when format is MuLaw

There appears to be an encoding issue with MuLaw. When playing back a generated tone, the audio is severely distorted. If the file is encoded in PCM then audio is fine.

Ex:
```csharp
var waveFile = new WaveFile(SampleRate.Hz8000, BitsPerSample.Bits8, DataChannels.Mono, WaveFormat.MuLaw);
DTMF.Generate(waveFile, TouchTone.GetTouchTones("1234567890"), 0.25D);
...
```