Practicing Should Not Be Hard Work
The point of these playbacks is to help you connect with your voice while practicing in a way that's more playful than serious. Although you should take your practicing seriously in that you want to warm up carefully and develop your vocal skills in a safe and productive way, you should not let yourself get bogged down with "right or wrong". Right is what feels and sounds good. Wrong is what feels like work and doesn't come easily. Always practice what you can, and leave the harder stuff to when you can do the easy stuff "in your sleep".
Some of these exercise playbacks are for those of you who have never practiced regularly, and some are for those who's voices are already well-trained and you're just looking for something new. Most of these exercises require that you've already warmed up your voice.
The Good News About Practicing
Never warm up your voice or otherwise practice for long periods of time. I'd say fifteen (15) minutes at most per session. Then give your voice a rest. I've known a lot of singers who think they have to set aside an hour a day for their practicing. This is fine if you're working on learning a song (after you've warmed up). But with "training", stay with short sessions and do a few throughout your day.
Be gentle with your voice on your warm-up exercises. Your singing voice is just waking up. Don't push it around. Don't force it to "perform". Don't sing loud. Don't expect too much. Your instrument is in your body and is subject to your life-style and eating/drinking habits, etc., so don't blame it if it can't do right away what it did yesterday when it was warm.
First session of the day: 2 to 5 minutes (while in the shower or making coffee, whatever). Don't worry if you're not happy with what your voice is doing. That's normal.
Professional singers who are singing on stage regularly (daily or weekly), don't need too much warmup because the voice is still warm from last night's performance. (In fact, often the voice needs a rest, so warming up might not be needed until shortly before tonight's performance.) I recommend starting with sirens and easy arpeggios using the "fluttering lips"or "blubbering lips" and sliding up and down your entire vocal range. Stop at any point when you feel tension. It just means your voice has reached its current limit. Never push your voice to do something it's not ready for.
As I've already said, about fifteen (15) minutes, or less, at a time. Then give your voice a rest (about ten minutes (longer doesn't matter; it can be hours). The voice gets tired easily when it's learning something it's never done before. You'll know when a rest is needed because you'll feel tension in the throat or mouth, shoulders or back, or even in the facial muscles.
It's different for all of us so the main thing is to know yourself when your voice has had enough. Anyway, the rule of thumb is: if it doesn't feel good, you need to stop (even if it's just for a while). If, when you start singing again, it still doesn't feel good, then it means you need a longer break. Simple.