While the tamagotchi music star is practising with his/her instrument, he/she will start making short noises once every few seconds, and while the sound is playing, a symbol of learning will appear above your tamagotchi's head.
This is a "learning stage" of your tamagotchi's practise time.
When this ouccurs, your tamagotchi is playing very well, and should be praised. In fact when "learning stage" happens, your tama needs the praise, and if you don't praise it, the stress level will go up quite a bit more than normally, quite like a small care miss.
You need not praise your tamagotchi just because he/she is practising, but when he/she enters learning and starts to make noises while practising, you should praise.
It is perfectly normal that your tamagotchi keeps practising after you praised, but the noises will stop.
Learning stage will also ouccur when you manually make your tamagotchi practise, so every time you choose "practise", you should also praise your tamagotchi just afterwards.
Apart from being required in learning, praise will also lower the stress level, so it's better to praise too much than too little.