There's no right or wrong way to format your radio script. However, some formatting is better than others.
At the end of the day, you want the best possible performance from your voiceover artist, whilst keeping direction clear.
Here's how I do it.
MVO: Male Voiceover
FVO: Female Voiceover
SFX: Sound Effects
Every writer formats their radio scripts differently. Find what works for you and run with it!
I can write radio scripts for you. Contact me with any freelance opportunities here. And check out some of my work here.