My 2 cents. I had this same issue with CM7. Every build I tried, my stock earbuds would not work for making phone calls. Every time I plugged it in and made a call, it would activate my speaker phone. The earbuds worked fine on stock, Icarus, CM6, and Froyo 2.2.2. I only ever had this issue on CM7. This was the biggest reason I did not like CM7.
Knowing that the Huawei Ascend has several different hardware configurations, it's not to far fetched to think that what may work for someone will not work for someone else.