It doesn't make any difference how you bought your tv - online or instore - the Sale of Goods Act applies.
The Act states must be three things:
As described
Fit for the purpose
Satisfactory quality
Unless it is advertised as having 'excellent' or 'superb' (or similar) sound then it is as described - you have the set you ordered. If you bought it because it was so described, you are entitled to reject it for a full refund.
Is it fit for the purpose? If you can watch it and hear it then, yes, it is.
Now, satisfactory quality is a grey area. The courts take in to account things like the price paid. If this is a low spec, cheaper model then you cannot realistically expect it to have top end sound. However, if the sound is so poor you cannot enjoy the use of it, then clearly it is not of satisfactory quality.
Look up the reviews online for that model tv - is there any mention of poor sound reproduction? If there is, you can use that to prove the television is below the standard expected.
In the first instance right to the company you bought it from and tell them the set is not of satisfactory quality because of the poor sound reproduction, you believe this to be an inherent fault and you want a full refund.
Keep a copy of all correspondence and send it by recorded delivery.
If you do not get satisfaction from your first letter, sent a Notice of Intent, stating you will pursue it through the County Court using the Small Claims' Procedure if they refuse a refund.
If you have to, you can start the proceedings online. It is inexpensive and relatively easy. Be prepared to pay an independent expert to provide evidence that the television is below the standard one would reasonably expect.