From my understanding of your statement of facts, the mold was discovered in the basement wall when your current contractor was drilling into it to install a dryer vent in the new addition. You want to know if you have any legal recourse against the original builder.
There are several issues here. The first is how long ago the original builder built the house. Assuming that the original builder used wet lumber, or otherwise was negligent in the construction, you potentially have a claim against the original builder for breach of warranty and negligence. However, the statute of limitations
on these type of actions are generally between 2-4 years. You might have a case where the statute of limitations does not start to run because you could not "discover" the damages of the negligent act until you took the wall apart. So, you potentially do have a claim.
The second issue is that you have not really established the cause of the saturated lumber. The fact that it is in a basement wall means that there could be a number of reasons that caused a leak, which might not be related to construction flaws. Having established the damages, you need to find someone to come in and make an initial determination as to the cause of the wet lumber. If they determine that it is related to a construction flaw by the original builder, then you should contact an attorney to present your claim.
Please let me know if you need further clarification.
Zachary D. Norris