Some fans described the sound quality at Spice Girls gig in Dublin as 'horrific'.
Mel B has promised the sound quality will be much better for the Spice Girls concert in Cardiff.
The first night of their reunion tour has left fans at Dublin's Croke Park disappointed with many saying they couldn't tell which song the girls were singing.
Mel B, Mel C, Geri and Emma hit the Principality Stadium on Monday.
One fan wrote on Twitter: "There's something wrong when the crowd at a Spice Girls concert are all sitting down because no one has a clue what song is on because the sound really is that bad."
Another concertgoer tweeted: "We cant hear a thing sitting up the top row, up the back, all we're getting is mumble, and for €110 a ticket, not good!"
One fan described the sound as "horrific", adding: "Even now when the girls are just talking you can't hear them properly."
After the show, Melanie Brown - known as Mel B - acknowledged the sound problems as she thanked fans for attending the Dublin concert.
She said on Instagram: "We will see you in Cardiff. And hopefully the vocals and the sound will be much, much better, pfft."
Brown, Emma Bunton, Geri Horner and Melanie Chisholm - aka Scary, Baby, Ginger and Sporty Spice - are performing 13 dates on the Spice World tour.
It is the first time the girls have reunited since their performance at the closing ceremony for the 2012 London Olympics.
Taoiseach Leo Varadkar was spotted among the 82,000-strong crowd at Croke Park, where Horner greeted the audience saying: "Welcome to Spice World. Spice girls, spice boys, everyone is welcome.
"We want every single one of you to feel special tonight. Like a king or a queen, we celebrate you. But I got to say there are a lot of queens here tonight."
Mel C told the crowd it was very special for the band to be back in Dublin.
She said: "In '98, we started our world tour in Dublin so we truly are home. It's got a very special place in our hearts."
The stage, which featured an imposing Spice World sculpture and large screens on each side, was used to display messages of female empowerment, including "We are stronger together" and their motto "Girl Power".
They opened the concert with one of their biggest hits, Spice Up Your Life, and a fireworks display.
Fans were spotted recreating the group's famous outfits, including Horner's union flag dress, Bunton's blonde pigtails, and Mel B's leopard prints.
Beckham - aka Posh Spice - decided not to join the current tour but she sent a message of encouragement to her former bandmates.
After their Dublin show, the Spice Girls will perform concerts in Cardiff, Manchester, Coventry, Sunderland, Edinburgh, Bristol and London - finishing at Wembley Stadium on 15 June.