We live near a country pub. Most of the time it is very quiet. However they have installed a speaker in their beer garden and smokers area, which now plays constant pop music during busy periods. Whilst not particularly loud it is intrusive, especially when coupled with the voices. We have rung and asked them to switch it off but they hang up.
If they have ignored you you can go directly to your environmental health department. Try also your licensing officer as there may be a condition on their licence about music in the garden (you can ask them for a copy of the licence by email). In any case one or both should be able to encourage a positive response. If not there are formal routes that may be explored.