But then how do I go around with the measured values to determine the sound pressure in dB SPL?

Attenuator meaning the knob which is used to control the volume. I have measured voltages at different values on the knob.

This means at 0.5 Vrms, it should be 96 dB. However, when I hear the audio or tone. It doesn't sound as if its 96 dB SPL. It is at comfortable normal conversation level. Do I need to subtract the value in dB of the attenuator too?

I just edited my question.

Thanks for your answer. So when I measure 1 Vrms going through my headphones that equals 102 dB. My sound card (headphone amp) also mentions that Voltage gain = +6dB (optimised for loudness). Does this mean when I measure voltage or rather when the calibration tone (1 Khz) is played out, it has a gain of +6 dB.

I also checked the specs of sound card headphone amp and it mentions that output impedance <30 ohm (unbalanced), then why do you I get high values?

Thanks for the answer. But this output impedance is a variable value. Should it be like that?