Most of the time girls will not give you IOIs. You must learn to run game on a brick wall.
The biggest IOI is simply eye contact and her attention. You should be getting that occasionally after approaching.
If you're never getting it, it's because your vibe is off: you're full of fear, you aren't relaxed, you're stuck in your head, you aren't smiling, you aren't in a good mood, you aren't having fun, you aren't direct enough with your sexual intent, you are too meek.
As you approach, command her attention. Force her to give you here full attention or blow you out. Do not do half-assed, meek approaches. When you approach, approach with 100% commitment and face her directly, with full eye contact.
It's okay if she blows you out. What's not okay are meek approaches.
The only importance of IOIs is to immediately capitalize on them as soon as you notice one. If you see a girl giving you eye contact, immediately approach and rapidly escalate. But never wait or hope for IOIs. IOIs are like finding cash on the ground. As soon as you see it, you grab it. But you don't rely on this to make money.

You can just train your voice in the shower. Singing is good, speaking is good, pronouncing words out loud is good. There are many vocal exercises you can do. There are even speak coaches. Roger Love is a great one. He has courses and books.