Learn Italian: Sento il chitarrista accordare la chitarra, infatti è pronto. - I hear the guitarist tuning the guitar, in fact he is ready.

In Italian, verbs of perception like sentire (to hear/feel) and vedere (to see) are followed by an infinitive when describing an action you witness happening. While English uses the present participle ('I hear him tuning'), Italian uses the infinitive construction (sento ... accordare).

Could I use an explicit clause and say Sento che il chitarrista accorda la chitarra instead?

Yes, that is grammatically correct and means the same thing. However, using the implicit infinitive construction seen in this sentence is much more concise and natural for verbs of perception, similar to the implicit indirect questions you are currently studying.

Does infatti mean the exact same thing as 'in fact' in English?

They often overlap, but infatti is a tricky partial false friend. In Italian, infatti is strictly used to confirm, explain, or reinforce what was just said (like 'sure enough' or 'indeed'). In English, 'in fact' is sometimes used to contradict a previous statement. To contradict something in Italian, you must use in realtà (actually), never infatti.

Is the verb accordare only used for musical instruments?

While its primary meaning in everyday language is 'to tune' an instrument, accordare is also used to mean 'to grant' (like accordare un permesso, to grant permission) or 'to harmonize' (bringing different opinions into agreement).