HAS TO BE PRESENT

Well then... if the spiritual master has to be physically present...
how do you explain this?

"His [Lord Brahma's] acceptance of the sound [tapa] was due to
his pure vision of the absolute nature of the Lord. And due to
his correct vision, he made no distinction between the Lord
and the Lord's instruction. There is no difference between the Lord and sound vibration coming from Him, even though He is not
personally present. The best way of understanding is
to accept such divine instruction, and Brahmā, the prime spiritual master
of everyone, is the living example of this process of receiving
transcendental knowledge. The potency of transcendental sound
is never minimized because the vibrator is apparently absent. Therefore 'Śrīmad-Bhāgavatam' or 'Bhagavad-gītā'
or any revealed scripture in the world is never to be accepted as
an ordinary mundane sound without transcendental potency." SB 2.9.8

Here Srila Prabhupada states, "the vibrator is apparently absent."
In the case of Lord Brahma he has not disappeared.
We may not be able to see Him from our present position within
the material creation but He is still present just as He was
at the beginning of the creation. Actually his lifespan covers the entire time of the manifestation of the creation.
When He leaves his body the universe is unwound.
So he is manifest "physically" the same now as then.

Lord Brahma is referred to as "the prime spiritual master of everyone."
He has not disappeared in the sense of leaving his body.
He is residing on Brahma loka in the upper planetary system.
Simply we cannot see him from here.

"The potency of of transcendental sound is never minimized
because the vibrator is apparently absent."
It is... the potency of transcendental sound... that is being discussed.
That transcendental sound vibration is being presented by
different realized personalities all throughout time.
We are currently receiving the mercy of the Brahma sampradaya an unbroken line of spiritual masters coming from Lord Brahma himself
of which our Srila Prabhupada is a most recent empowered acarya.

Anyone and everyone benefits from giving submissive aural
reception to the messages of Godhead coming from Lord Brahma.
After all Lord Brahma is the prime spiritual master and he is
physically present on Brahma loka. He is incarnate.

So that's all there is to it.

But this is easier said than done. Krishna is fully present in His name too.
But who is so advanced that they are fully realizing that at every moment.

So there is a process. Bhakti yoga. In order to practice the process
of bhakti yoga a person will need to approach a bonafide teacher
of bhakti yoga, referred to as "a bonafide spiritual master" in line of disciplic succession.

There really is no way of getting around this.
If someone cannot fully surrender and seek out a
bonafide spiritual master, who is physically present and
teaching bhakti yoga, the benefit is still there by practicing
as best they can from wherever they are situated in the material condition.

But to be very sure to avoid offenses to devotees
who are teaching the process.