Music in the House of Worship is to be vocal only, whether by singers or a singer. It does not matter if a guest a cappella choir, or soloist is used, provided such use is not made the occasion to publicise services of Worship and the precautions you mention are taken. No doubt the excellent recordings available today would assure the highest quality of performance at low cost, but all references to vocal music in the central Edifice imply the physical presence of the singers.
In a letter through his secretary to a Chicago believer in 1931, "Bahá'í News", No. 55, page 4, Shoghi Effendi expressed the hope that "... now that the Temple is completed it will be filled to the full with pure seeking souls. It should be different from the other houses of worship which even if they are filled, their source of attraction is the music heard. Here the spirit should be so powerful as to awaken the heart of every one that enters it to the glory of Bahá'u'lláh ..."
- The Universal House of Justice (From a letter dated 13 March 1964 to the National Spiritual Assembly of the United States; The Compilation of Compilations, vol. III, Service in Baha’i Temples)
