Not only is it in bad taste, but it’s illegal. Also, remember you’re supposed to be paying attention to your playing partners so you can sign off on your marker after the round.
I think the rule focuses on “briefly” vs. “prolonged”. If you’re planning on listening to music on your headphones I would certainly call you on it.
Wearing Headphones or Earplugs During Stipulated Round
Q. May I wear headphones or earplugs while playing in the state amateur?
The use of headphones or earplugs to eliminate noise or other distractions is prohibited under Rule 14-3.
Decision 14-3/16 covers listening to music or some other broadcast during a round. Here is the text of that Decision:
Rule 14-3a states that a player may not use any artificial device or unusual equipment that “might assist him making a stroke or in his play.” Listening to music or a broadcast while making a stroke or for a prolonged period might assist the player in his play, for example, by eliminating distractions or promoting a good tempo. Therefore, the use of an artificial device to listen to music or a broadcast, whether or not through headphones, while making a stroke or for a prolonged period of time during a stipulated round is a breach of Rule 14-3. However, it would not be a breach of Rule 14-3 for a player to listen to a device briefly, for example, to obtain the results of another sporting event or traffic information, while walking between the putting green of one hole and the teeing ground of another hole.
There is no restriction on listening to music or other broadcasts while practicing (whether on the practice ground or on the golf course, and whether by oneself or while playing with others), although club rules and disciplinary codes could apply in such circumstances.