Besides the Nādabindūpaniṣad, I came across other interpretations of how praṇava is manifested from mantra sohaṃ. There is also the interpretation in which, if we remove ‘sa’ and ‘ha’ from sohaṃ, we get praṇava Oṃ. One of the early texts that I went through was the Prapañcasāra Tantra (4.21):
सकारं च हकारं च लोपयित्वा प्रयोजयेत्।संधिं वै पूर्वरूपाख्यं ततोऽसौ प्रणवो भवेत्।।४.२१ ।।
sakāraṃ ca hakāraṃ ca lopayitvā prayojayet ।
saṃdhiṃ vai pūrvarūpākhyaṃ tato’sau praṇavo bhavet ।।4.21।।
According to the image of pūrva rūpa saṃdhi, removing ‘sa’ and ‘ha’, one must definitely get that praṇava (oṃ) from there.
Explanation: pūrva rūpa saṃdhi is connected with the ending ओ/o, which stands before अ/a of the next word, turning into a sign known as avagraha / ऽ. Thus, the text implies that from ओ/o we get the praṇava mantra.