Yes, that is possible. Knowing that the remasters were being done, Sony/BMG probably did not restock these titles once the existing stock sold out. This has happened from time to time whenever any of the labels are planning on remastering and reissuing older catalog titles. The discs are not withdrawn from stores, but the title is notated not to have any additional copies made.
When a title is being completely withdrawn from the catalog a notice is sent to stores to recall all stock copies (if there are any).) This is done so stores can receive full credit for the discs. (Whatever copies are returned are drilled or sawed and sold off as "remainders" which turn up in discount bins.
Cast albums are NOT "soundtracks."
Live theatre does not use a "soundtrack." If it did, it wouldn't be live theatre!
I host a weekly one-hour radio program featuring cast album selections as well as songs by cabaret, jazz and theatre artists. The program, FRONT ROW CENTRE is heard Sundays 9 to 10 am and also Saturdays from 8 to 9 am (eastern times) on www.proudfm.com