Lumia 830 has louder and richer volume output than Lumia 925 and it has “Dolby surround sound” support even without headphones, while Lumia 925 has Dolby support with headphones on.
You may like to read our Lumia 830 hands-on preview and full unparalleled coverage of Lumia 830 by clicking related links below,
Read Lumia 830 hands-on preview
Lumia 830 outdoor display test under direct sunlight
Lumia 830 bend test and knife test
Lumai 830 Dolby surround sound capture samples,
Read, Can Lumia 830 replace my Lumia 925 / Lumia 920?
Read our Lumia 830 and Lumia 925 display comparison.
Read Lumia 830 vs Lumia 925: Camera comparison in daylight stills and low-light stills and video capture shootout for more.
Read our Lumia 830 vs Lumia 925: Hands-on performance test, multitasking, Camera speed, browsing video + Benchmarking comparison.