refactor: A few changes to the audio preview and visualizer

• Set reasonable size for audio visualizer: For the voice recorder it would be great to deduce the optimal width by the popover size during runtime. This should be investigated later.
• Audio visualizer: Remove css class and adjust comment
• Visualizer: Guard against division by zero and account for empty samples
• Log errors as error not as debug or warning messages

Please see the commits for details.