Fix IndexSizeError by checking if audio is seekable

Add a check to first verify that the current audio is seekable before attempting to call `seekable.end(0)`. Fixes this error:

![screen shot 2014-10-13 at 5 52 24 pm](https://cloud.githubusercontent.com/assets/2916671/4620886/24f6c878-5324-11e4-8722-f9f3bf276f18.png)