Incapsulate logic of getting file url in Sound manager

[ViktorVovk]

This PR is intended for a minor refactoring of the howler-sound-manager.

• Two private methods, _getSoundUrlsFromResource and _getDefaultSoundUrl, have been created to handle the formation of src.
• In the loadResource method, the preloadAudioFile function was being created each time, despite not using any data from the closure. The preloadAudioFile function has been extracted as a private method.

This minor refactoring can help extend the functionality of the class by inheriting from SoundManager without the need to override much of its implementation.