For most people Agnetha Fältskog (the blond soprano) was considered the most prominent singer of ABBA. However the alto of ABBA, Anni-Frid "Frida" Lyngstad, could hold her own. She was/is a very versatile singer. In fact before ABBA Frida was a jazz singer in Sweden.

Here's a clip of Frida from 1967 before ABBA singing a smooth jazz song in Swedish which includes some scat singing segments. The song begins at 1.50 after an interview.

Frida was challenged to try singing opera during a televised jam session with ABBA, Andy Gibb and Olivia Newton-John. Although she hesitated and although she more or less was joking there's no doubt she had potential. Here's the outcome

Here's Frida singing Dancing Queen acapella with "The Real Group".

Here's Frida doing a little yodeling