Learn Spanish (Spain): Oí a alguien tocando la melodía en el piano. - I heard someone playing the melody on the piano.

This is the core of your current lesson! Using the gerund tocando focuses on the action in progress. It means you caught them right in the middle of playing the melody. If you used the infinitive tocar, it would usually imply you heard the entire performance from beginning to end.

Why is there an a before alguien? Don't we only use the personal a for specific people we know?

Normally, yes, the personal a is for specific, identifiable people. However, the pronouns alguien (someone) and nadie (no one) are major exceptions. Because they inherently refer to human beings, they always require the personal a when used as a direct object, even though their exact identity is unknown.

Why does it say en el piano? Shouldn't 'playing the piano' just be tocar el piano?

You are right that 'to play the piano' is simply tocar el piano without any preposition. However, here the verb tocar applies directly to la melodía (playing the melody). The piano is the instrument where the melody is being produced, so we use en el piano to mean 'on the piano'.

If I wanted to replace a alguien with 'him', would I use lo or le before the verb?