How to use correctly use 是否?
是否 roughly maps to whether in a yes-no interrogative sentence or clause.
For your sentence, you should use 不知道 or 不确定 instead of 没确定. 没确定 means haven't confirmed yet. Also 听得清 (could hear) is more polite than 听清了 (have heard). If the context allows, it is more natural to omit 我说的话.

I wasn't sure if you could hear me clearly.
我不确定你是否听得清(我说的话)。

General usage of 是否 is pretty straight-forward. You can check out Iciba for examples.