I usually just sing, with no cd. My kiddos usually sing with me, although they sometimes need encouragement to do so. We do a lot of movement songs too and they don't usually sing through those because they're usually so focused on the movements.
To encourage them to sing I sometimes start circle time with "warming up our voices", where we lightly tap our throats and make silly sounds (like, eeeee-oooo--eeeee-ooo....). I tell them that this helps them to remember to sing...lol. It usually works.
I always let them choose the songs we sing too, which helps.
Also, sometimes we'll sing a song loudly (like, totally screaming) and they LOVE that. We also sing songs in a whisper, which they sing.
I also allow them to sing a song for the whole group if they want. They really like that, although sometimes it goes on longer than the other kids like
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Sometimes I'll pretend to sing a song and not know the words (I do this with the ABCs a lot), then I tell them that they should sing it without me so I can learn the words.
I've definitely found that by varying things, they're more likely to sing along. I have 3-4 year olds.