What kinds of foods or drinks soothe your voice or make it sound better when singing? For example, I heard lemons help.

Water is excellent. Thins the mucus and lubricates the vocal chords. Reduced friction of chords equals longer lasting voice.

https://www.schoolofrock.com/resources/vocals/7-tips-on-how-to-keep-your-singing-voice-healthy

Many singers have their own recipes and rituals involving diets they'll stick to to retain their good singing voice. I can't provide an internet link to back up my own - but I've always found gefilte fish (cooked Hungarian style with plenty of broth) to be an ideal agent of good vocal production. For the one thing, it's various fish oils soothe the throat well enough, and it makes one thirsty, which causes one to drink more water, which (as has been stated) is good for the vocal cords.

There are a few foods that soothe the throat such as candy or licorice tea. Drink lukewarm drinks such as herbal tea or water. Some healthy meal choices for your body and voice include chicken, almonds, fish, yellow vegetables, apples, eggs, and rice.
Salt water or baking soda gargles, sugar lozenges or some hard candies, and honey can help soothe the throat if it's irritated just before a performance.

http://www.33rpm.com/Foods-to-Eat---or-Avoid---Before-Singing-82-art.html

When I was younger and "in voice" on our way to estedfodds (spelling) our teachers always advised us to eat pineapple saying it was good for voice projection.