The reason we typically do not observe interference effects between light beams from two flashlights or sound waves from two violins is because the light and sound waves emitted by these sources are generally incoherent.
In order to observe interference, the waves need to be coherent, meaning they have a constant phase relationship. This coherence allows for constructive or destructive interference to occur when the waves interact. However, in the case of flashlights and violins, the sources emit light or sound waves randomly and independently.
Flashlights emit light waves from separate sources (bulbs) that are not synchronized with each other. Similarly, when multiple violins are playing, each violin produces sound waves independently without any coordination. As a result, the emitted waves have random phases and do not maintain the required coherence for interference to occur.
In contrast, interference effects can be observed in situations where coherent sources are used, such as in experiments with laser beams or in musical instruments like pipe organs or string ensembles. In these cases, the waves are produced in a coordinated and coherent manner, allowing for interference patterns to emerge.
It's worth noting that even if two flashlights or two violins were somehow synchronized, the wavelengths of visible light and audible sound are quite different. The wavelength of visible light is on the order of hundreds of nanometers, while the wavelength of audible sound is on the order of meters. This difference in scale makes it difficult to observe interference effects between light beams or sound waves without specialized setups and careful control of the sources.