Shazia Ahmad and Wasim Ali, from Newport, Umar Arif, Umar Butt and Khalid Yassen, from Cardiff, and Zawed Malik from Greater Manchester, all deny conspiracy to supply the class A drug.
Cardiff Crown Court heard two cars were followed by undercover police officers.
Almost 40kg (88lbs) of heroin worth about £5m was seized between 2013 and 2014.
The jury was told the speaker was being carried in a car that was stopped by police.
Mr Butt and Mr Yassen were arrested on the M5 with 36.5kg of heroin in a speaker and suitcase in their Citroen Picasso.
Undercover officers said they followed the Picasso and a Fiat Punto "travelling in convoy" but admitted under cross examination that they did not see a "speaker box" being placed in the Picasso.
The court heard money was handed over at locations across Newport, including a Lidl car park.
The case continues.

5 February 2016 Last updated at 15:27 GMT
This year the festival features over 200 sculptures including iconic Asian buildings carved out of snow and ice.
A  ten metre tall model of a high speed train is also one of the big attractions.
Organisers hope over two million people will visit from all over the country, and abroad, to see the sculptures.

Felix Barrow sustained many broken bones and spent three weeks in a coma when he was knocked over outside his home in Swanmore in October last year.
His parents were told he might never walk again, but they said he has now exceeded all his doctors' expectations.
The family has raised nearly Â£100,000 for the services that saved his life.
Felix was on his way to school when he tripped in the road and was hit by a car as he was getting to his feet.
His mum Vanessa said the accident was "horrific" and added: "You just think this can't be happening to you.
"It was just a normal Wednesday, normal things going on, and your life is turned upside down into an unrecognisable sate where you are thrust into a hospital environment.
"At first I don't think we really realised the extent of his injuries.
"It's been a slow process of learning to do things again, learning to walk, learning to eat, drink normal fluids."
She added that her son has surpassed all expectations and made "amazing" progress.
Felix's dad Stuart said his life was saved by the Hampshire and Isle of Wight Air Ambulance, which flew him to hospital, and the Southampton General Paediatric Intensive Care Unit.
He said: "We wanted to repay them for saving his life. It's been fantastic, the whole community has come together."
The family set up a charity, Fit For Felix, inspired by his love of running and named by his friend Bradley.
It has now raised Â£93,850 by organising a range of activities including fun runs, bike rides, and sponsored swims.

Abdou, who joined Millwall from Plymouth in 2008, signed a new one-year deal to stay with Lions until the end of next season on Thursday.
The 33-year-old made 22 appearances for Millwall last season, helping them win promotion from League One.
Abdou is Dons manager Neal Ardley's fifth signing since the end of last season.
Find all the latest football transfers on our dedicated page.
Six people accused of being involved in Wales' largest heroin seizure kept the drugs in a speaker, a court has heard.
[NEXT_CONCEPT]
A snow and ice festival in Northern Japan saw some pretty impressive sculptures made out of the cold stuff.
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A 12-year-old boy left with a severe brain injury after he was hit by a car has returned to school after making an "amazing" recovery.
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League One side Wimbledon have signed Millwall midfielder Jimmy Abdou on a season-long loan deal.