Changed a little source code to support to daisy 3.0
Fixed 3 cases
1. In case 1 chapter has 2 audio files. While audio 1 is being played at the end, pressing the button Next sentence to move to the first sentence of audio 2
2. In case 1 chapter has 2 audio files. While audio 2 is being played at the begin, pressing the button Previous sentence to move to the last sentence of audio 1
3. Two text and have one audio file