As a musician, listening back to your sound, and evaluating what aspects of your performance and practice could use more attention, can be a valuable use of your time.
Decades ago, it was common for musicians to hear recordings of their live performances. Much as a basketball team might review videos of their previous night’s game, taking notes and making adjustments, a musician and teacher can share notes on phrasing, intonation, articulation, and other aspects of the performance.
Now that recording gear is so inexpensive, it does not require much in the way of equipment to document your playing. You can record yourself on your phone’s Voice Memos in your practice room. Or, if you would like higher sound fidelity, you may consider buying a small professional recorder, just as a Zoom H1, a Yamaha Pocketrak, or a Tascam DR-05. There are many consumer models that are comparable in price and value.
When recording yourself, it is often best to position the recording device across the room from you. That way, you are hearing how you sound in the room, and using the room’s natural acoustics to your advantage.
When you give yourself some room from the recording equipment, you also can avoid some of the distracting instrument sounds that are not (necessarily) part of the musical expression: clacking keys on a woodwind, fret noise on a guitar, or string noise on a violin or cello.
Being able to evaluate your playing after the fact is a great skill to develop. While you are in the performance, it is difficult (if not impossible) to truly gauge how your sound is filling the room, or how your phrasing feels.