Perhaps, Manu 4.125 may indeed mean "extracted from the Veda" to refer to OM. Cf. Taittiriiya-Upani.sad, Ziik.saadhyaaya 1.4.1: yaz chandasaam .r.zabho vizvaruupa.h / chandobhyo 'dhy am.rtaat sambabhuuva //. Here Zankara takes these lines to refer to OM as the extract of all the Vedas, the am.rta. Perhaps Manu has some such notion in mind. Best,
Madhav Deshpande