You should have downloaded the MIDI file of the final chorus. (Alternatively you may have received an audio CD containing all of the MIDI files for the Basses and Sopranos, in which case this sound file comprises track 15.) The MIDI file corresponds directly to the score, starting from Page 35 at bar 910, on which page the entire spoken text preceding the final chorus is reproduced.
The MIDI file contains the individual vocal part in Oboe voice in the left stereo channel, with the piano accompaniment and full chorus sounding as Acoustic Grand Piano in the right channel. This allows you to control the "mix" of vocal part and accompaniment by using the balance control on a stereo system. A click track is not provided as this is often inimical to the complex rhythms; in the actual concert you will still have to count, and without a click track for a crutch!
The choice of oboe voicing is because the "Vox humana" MIDI sounds can vary largely from system to system; the tone of the Oboe voice is usually strong and clear throughout its tessitura ranging from high soprano to low alto notes; and the Oboe sustains unlike the sound envelope of the Acoustic Grand Piano, which decays immediately after the initial attack.
The new Collected Edition of Schönberg’s works published in 2002 has correctly numbered bars from figure 39 onward; at some point after 1911 Schönberg compressed two bars in the original version into one bar (the current bar 389 with 3/4 time signature) with the result that only 9 bars separate figures 39 and 40, and figure 40 is really bar 399 rather than bar 400. All of the subsequent bar numbers given in this edition are one greater than the numberings in the Collected Edition.
These bars may be sung by a semi-chorus, but the composer does not specify whether the sole soprano part is to be sung by either first or second sopranos. All of the 1st sopranos rejoin at bar 946.
The final word, Pracht, may be expected to last a very long time (up to 22 seconds, with a crescendo, in one popular recording). Stagger breathing. The practice of sending the first sopranos up to top C at bar 1040 is not based on the authority of any of the manuscript or printed full scores.