Two instruments that sound like a voice to a dog
Your dog is probably not reviewing the music. A harmonica and a recorder share something most household music does not: a pure, breath-driven tone that rises, holds and wavers in a range close to a howl. To canine ears, a long recorder note can resemble a far-off dog calling across distance, and the instinctive reply is to answer. Guitars, piano and radio songs layer many pitches at once and change quickly, so there is no single sustained call to answer. That explains the odd selectivity you are hearing: he ignores a whole playlist, then lifts his nose the moment a reed or wind instrument holds its first long note.
Joining in is social, not usually distressed
Watch his body while he howls at practice. A dog who is answering socially usually looks loose and interested, with a raised head, soft ears, a wagging or gently sweeping tail and pauses to listen between howls as if waiting for a reply. He may move closer to the player, because the sound is coming from a favourite person and joining in has become part of the ritual. That is different from a dog who howls with a tucked tail, pacing, lip licking or attempts to leave the room, which would suggest the pitch is uncomfortable rather than inviting. The same instrument can produce both reactions in different dogs, so his posture matters more than the volume of the song he offers back.
Making practice work for player and singer
If the howling is charming and brief, you do not need to stop it, but give the player a way to practise scales without a duet when concentration matters. Settle your dog first with a chew or a sniffing game in the same room, because a dog whose mouth and nose are busy is less likely to throw his head back. Practise the highest, longest notes at a set point in the session rather than throughout, so the singing has a predictable window. Reward quiet listening during short phrases by dropping a treat between his paws without fuss, building the idea that listening also pays. If he gets overexcited, pause the instrument, wait for four paws and a breath, then resume more quietly. If more than one person practises in your house, expect different reactions to each player too, since timing, breath control and favourite status all change how inviting the call sounds.
When a musical howl is worth checking
A new sensitivity is the thing to notice. If a dog who once slept through recorder practice now howls, flinches or leaves whenever a particular pitch sounds, or if he shakes his head, scratches at one ear or seems sore around the jaw, have his ears checked rather than assuming a new musical taste. Likewise, if howling spreads to sirens, alarms and everyday sounds with clear signs of anxiety, note which sounds trigger it and how long he takes to settle. KinsLe makes that follow-up easier since Care Teams allows whoever was practising to record the trigger and his recovery time in one shared routine, giving you a clear account to share if you seek advice.