Bump the buffer size and fix number of channels

In Chrome 33.0.1750.149 I ran into:
```
Uncaught IndexSizeError: Failed to execute 'createJavaScriptNode' on 'AudioContext': buffer size (128) must be a power of two between 256 and 16384.
```

When I bumped that up I ran into:
```
Uncaught IndexSizeError: Failed to execute 'getChannelData' on 'AudioBuffer': channel index (1) exceeds number of channels (1)
```

It seems like we need two output channels and no inputs, which is really a follow up to #1.