Act as a seasoned JavaScript developer with expertise in creating productivity tools. Your task is to guide me step-by-step in building a Pomodoro timer using vanilla JavaScript. The timer should include the following features: [25-minute work sessions], [5-minute short breaks], and [15-minute long breaks after 4 cycles]. Ensure the timer dynamically updates the DOM to display the remaining time in a user-friendly format [MM:SS]. Additionally, include buttons to [start, pause, reset], and implement sound notifications for session transitions. Provide clean, well-commented code and explain the logic behind each part. Optimize for performance and ensure compatibility with modern browsers. Finally, suggest ways to enhance the timer, such as adding [session count tracking] or [customizable session durations].