"Esto me ayuda a oír mejor."
Translation:This helps me to hear better.
Why is the "a" needed
Since oír already means "to hear", what purpose does the "a" serve? It seems that sometimes the "a" is needed before the infinitive and sometimes not. How do we know which it is?
In this case the 'a' is attached to the verb before it (ayudar) and not 'oír'. The phrase 'ayudar a' means 'to help to'. Remember you can't always translate every word literally. The fact that 'oír' means 'TO hear' is irrelevant. The 'TO' is only included because English doesn't really have an infinitive form for verbs like Spanish.
As for the second part, some verbs take a preposition (a, en, con, por, de) AFTER them to achieve a specific meaning. Another example would be 'dudar' + 'en' (to hesitate to). Unfortunately you just have to learn which verbs take which prepositions. There are lists online that can help.