An objective of the GOLF experiment (Global Oscillations at Low Frequencies) on-board the SOHO (Solar and Heliospheric Observatory) space mission was to measure the line-of-sight component of the disk-averaged magnetic field of the Sun. In this paper, a theoretical study of this observable is presented. The newly developed techniques are tested using simulated data series based on the observation of this experiment, and the sensitivity of GOLF to magnetic measurements is calculated. An average solar mean magnetic field of 0.120 +/- 0.002 G has been found corresponding to the minimum of solar activity by analyzing 26 days of full GOLF data. The power spectrum of this field is calculated and the main characteristics {are} described.