If your eldest is 4, I’d be surprised if you haven’t already received plenty of unsolicited parenting advice. I even had strangers on the street giving me advice, e.g. on how wrapped up they were.
If it appears well-meant, then take it in the spirit it’s given, smile and say: “Thanks, useful to bear in mind”. Take it into account, consider NHS advice and latest research. But make your own decisions. If she gets really persistent, you could say: “I guess we all parent differently”.
There’s no one way to parent. Everyone has conflicting views and things that have worked for them. “You should breastfeed as long as possible” - “You shouldn’t breastfeed more than 6 months”. Sleep training v co-sleeping. Weigh it all up, look at research. Then do it how you see fit.
As far as I know, a child won’t have a speech problem due to the tone their mother talks to them in. Your friend was being judgemental. I’m sure she wouldn’t like you judging her about how she parents her grown up kids or behaves with her partner.
The important thing is to talk to children. Talk through what you’re doing and why, what you see on walks, the colour of the sky and the shape of the clouds, pictures in books. Ask for their opinion, to help them learn: “Which is your favourite colour in this picture? Why’s that your favourite?”. Play role play games, like shops. These days, when so many parents and children occupy themselves with screens, chatting and doing things together is the most important thing.
I also used to have talk radio on in the kitchen: BBC 5 Live on weekdays. And I think hearing talking in the background may have helped mine a bit with absorbing speech.