Thank you so much for your comprehensive explanation of my questions. I think I have got my answers. I will try to dig down details how the TMRpcm library works and try to customize that as per my requirements.
I am not expert in creating and modifying Arduino libraries and my programming skills are in basic level. Would you please give me any reference link or guideline to understand the library and modify as per my need?
PD: to all this, why do you need to play four simultaneous audio tracks?
Actually, I am working on a haptics project. I need to design a vibratory haptic feedback system. I am using LRAs form Tactile Labs as the vibratory actuators. I need to vibrate those LRAs simultaneously and independently. I am using audio files that contain the vibration patterns for the LRAs. The audio files go out from Arduino board and then pass through an amplifier and finally vibrates the LRAs. To have the independent control of LRAs I need to play multiple audio files (contains the haptic pattersn) simultaneously and independently. I am using four LRAs right now so I need to play four audio files simultaneously.