Spoken Voice: Samantha Newark. Strangely enough Samantha did not sing for Jem, despite being something of a singer in her own right. 
Singing Voice: Britta Phillips. As well as singing for Jem, Britta is also bass guitarist for a band named "Luna" and did a few more acting roles besides.

Spoken Voice: Samantha Newark, as for Jem. 
Singing Voice (Yeah, Jerrica sings in Imagine Me, Something is Missing...) Britta Phillips. No difference between Jem and Jerrica (Except Jem generally sounds more vapid)

Spoken Voice: Cathianne Blore. Cathianne also voiced Aja, and Starlight girl Ashley. 
Singing Voice: Hard to say. Since the vocal for Im Okay belong to Stormer (see above), it's difficult to pinpoint a singer for Kimber.

Spoken Voice: Cathianne Blore (Like Kimber/Ashley) 
Singing Voice: I wish I knew - that snippet in Melody Playin' is enough to make anyone curious!

Spoken Voice: Cindy McGee. Cindy also did the voices of Starlight girl Krissie and of Sandra the urchin, far as I can tell, from Treasure Hunt. 
Singing Voice: Shana doesn't sing, and her single spoken line in "We're Up/You're Down" is more or less proven to be Britta.

Spoken Voice: Linda Dangcil
Singing Voice: Raya doesn't sing.