Hi @juragd
Does the volume change if you shake the laptop when listening to audio using headphones connected to the laptop?
If not try testing the Left and Right speakers in ''Control Panel > Realtek Audio Manager'' to see if only 1 speaker is the problem. If it is only one speaker then unfortunately they come as a complete module so you will have to replace both.
Here's the [https://download.lenovo.com/consumer/mobiles_pub/lenovo_g400g500g405g505g410g510_hmm.pdf|hardware maintenance manual] for the laptop.
Go to p.69 (73 of 108 pdf) to view the procedure to remove the speakers from the laptop. Initially you don't have to remove them, just check their connection to the motherboard is OK and that they look OK as well.
If it is determined that the speakers themselves are faulty search online for [https://www.google.com/search?client=firefox-b-d&q=90202714|90202714] to find suppliers that suit you best.