Hi Deep,
That depends on whether you are asking about a relative/absolute decrease of pitch or amplitude?
Might be of interest, if it's pitch:
Dixon Ward, W., & Burns, E. M. (1975). Pitch performance in singing without auditory feedback. The Journal of the Acoustical Society of America, 58(S1), S116-S116.
Mürbe, D., Pabst, F., Hofmann, G., & Sundberg, J. (2002). Significance of auditory and kinesthetic feedback to singers' pitch control. Journal of Voice, 16(1), 44-51.
Cheers,
Ole Adrian Heggli