In French lyric diction (poetry and art song), is the first vowel in "faisons" /œ/, as it is in everyday pronunciation?
[fə.zɔ̃] is the usual pronunciation in France. Faire has a highly irregular pronunciation. The letters ai are pronounced /ə/ in :
- the nous form of the present indicative.
- all the forms in the imperfect indicative.
In this other answer @Gilles explains that this strange pronunciation might have its origin in the fact that in the 17th century the people in Paris wrongly pronounced the present participle [fə.zɑ̃] and this pronounciation spread to those other forms, and outside Paris.
I gather in France the vast majority of people will pronounce [fə] and not [fœ]. I could not say about other French speaking countries.