Is there a way (or app) to input an audio file (for example a recording of my voice) into my microphone?
Yes, there is a way. You will have to get iShowU Audio Capture, Loopback, and an audio player. In Loopback you will select your audio player as a source, and then going past the output channels you will select iShowU Audio Capture as a Monitor device. This means that the audio from your audio player will get routed to the iShowU Audio Capture virtual driver. Now you can select iShowU Audio Capture as an input device (like a microphone) in your system's Audio Settings, and it will output the sound from the audio player.