Learn Swedish: Barnet hör gatumusikanten ropa till publiken. - The child hears the street musician shout to the audience.

Because hör is a verb of perception (a verb about hearing or seeing). Just like in English ("hears the musician shout"), when you hear or see someone perform an action, Swedish uses the object (gatumusikanten) followed directly by the bare infinitive (ropa) without the infinitive marker att.

In English, I could also say "hears the street musician shouting". Can I use a present participle here in Swedish?

You should stick to the infinitive ropa. While English distinguishes between hearing the whole action ("shout") and an ongoing action ("shouting"), Swedish generally uses the bare infinitive for both situations after a perception verb.

How is the compound word gatumusikanten formed?

It is made from gata (street) and musikanten (the musician). Notice how gata changes to gatu-. A small group of common nouns ending in -a change their ending to -u or -o when they become the first part of a compound word (for example, en gata becomes ett gatukök and en vecka becomes ett veckoslut).

Why is the preposition till used after ropa instead of på?