I’ve had postnasal drip/a slightly stuffy nose for half a year now, which makes my voice sound nasally even when I lift my soft palate. It’s also gruff and heavy. I’m trying to make it lighter and was getting somewhere before this stuffy nose. Advice?
Postnasal drip often causes irritation and slight swelling of the vocal cords. Many of us push into the cords in order to compensate and get an adequate phonation. This solution leads to the irritation and gruffness, the heaviness that you are hearing and feeling.
The better remedy is to use a very fine phonation, as though you are threading a tiny filament of voice through an opening in the congestion, like it is the eye of a needle. You never resist or push against the phlegm or the voice. All of the work is in keeping the ribs expanded so that as little air as possible is passed through the vocal cords.
Once the sound is refined enough, you can use humming and vowel chanting to increase the overtones. Once the stress is relieved you should be able to access all of the resonators in the chest and head and the nasality should disappear.
When you are using less air, a more refined phonation and balanced resonance, the sound will naturally be lighter, more flexible. It will also be louder and more beautiful without even trying!