Welcome ro the forum.
What makes the sound of your MIDI notes has a lot to do with how realistic or artificial it sounds. The computer's sound card synthesizer, Microsoft's GS Wavetable synthesizer is infamous for it's unrealistic sound, but is easiest on the computer's processor.
You could have your MIDI output set to an external keyboard or sound module. Many of these are of high quality, such that it's hard to tell the difference from a real instrument. Very little impact on the processor.
Or you could use a software synthesizer, such as Cakewalk's TTS-1 or SoundCenter. These synths sound pretty good, but can use a lot of processing power, which could cause noise, jerky sound or no sound at all (audio dropout), especially when using a computer's built-in sound card.
An audio interface designed for recording and processing sound is the solution to noisy playback and dropouts.
What are you now using to make your MIDI notes sound?