Is it permissible to vocalize during supererogatory (nafl) prayers if that increases one's devotion (khushu')?

The default for nighttime supererogatory prayers is to recite aloud, and for daytime supererogatory prayers, it is to recite silently. It is disliked to recite aloud in supererogatory prayers during the day, except for solar eclipse prayers and prayers for rain (Istisqa'). As for nighttime supererogatory prayers, the benefit (maslahah) should be considered: one should recite silently if someone is bothered, and recite aloud if someone benefits. There is no disagreement regarding the desirability of reciting silently in absolute daytime supererogatory prayers. There is no harm in reciting aloud in a daytime supererogatory prayer if it leads to greater devotion (khushu'), but silence is preferable. Ibn Baz permitted raising the voice in supererogatory prayers at night if it led to greater devotion for the worshiper and did not bother anyone. As for daytime prayers, the Sunnah is to recite silently, though the Imam may occasionally recite some verses aloud.