How many BTU per cubic meter does typical paper waste yield when burned in a modern domestic fireplace
About 12 million BTU, or 3.5 MWh.

I searched for paper btu and got loads... of... hits.

Seems like 7000 BTU/lb is a decent starting point, or $1.6 \times 10^7\ \mathrm{J/kg}$. If we say a ream of letter-size paper weighs about 5 lb, then we can calculate that paper yields about $1.2 \times 10^{10}\ \mathrm{J/m^3}$ when burned — enough to heat the average US house for 15 weeks.