Source: Vijay B
I asked my Guru (mukkUru Lakshminarasimhachariar) about having learnt many mantras just from attending yajnas and hearing them. He told me that if you have heard the mantra in a natural setting (say being recited in a yajna or being recited by vedavits) and learnt it from there, then it is equivalent to learning it from a Guru directly by upadesha.
He also said that if one learns non-traditionally etc. then they will never shine in a sabhA. अन्तः गुडुगुडायते बहिर्वक्तुम् न शक्यते। So this is the necessary side-effect 🙂