If you want to see analog or digital signals there is always an oscilloscope where you can choose various parameters (scale, time). But if you are building a project where you only need constantly to monitor some known signals like audio or sensor data, then oscilloscope would be an overkill. Why not to include TFT LCD display module to your project and have visual feedback on what’s going on.
dmainmon in his instructable have shared his demo where he used Arduino Mega 2560 R3 and 2.8” TFT LCD display to display the analog signals of your choice. In his demonstration he used couple signal sources: light sensor and audio sensor. The graphs simply compile on LCD display continuously. He implemented simple touchscreen menu where he can select some properties on how those graphs are displayed. He can select graph color and sweep direction.