That's easily explained. SFX are very memory hungry on the Amiga since they samples and need to be in Chip RAM if you want to use them through DMA access (which is fast).
An Amiga 500 for which this game is aimed at has only 512 kb of Chip RAM, and Inviyya uses almost every little piece of that for fast triple buffering, music, multi parallaxing, keeping all kinds of gfx and animations that are needed all the time (fonts, explosions, the players ship and its shots, upgrades, etc)... 512 kb might sound like a lot, but all that stuff adds up quickly.
One other explanation for sfx missing for some things is simply a lack of time. In the end I just wanted to get the game shipped after months and months of bug fixing and improving stuff.
I concur, SFX are lacking in the game, but if you are just one person who does EVERYTHING it all comes down to compromises in the end if you want to create a game that rivals old titles from back in the day.
Hope you will like the game anyway, mate...
Thanks for your already written and hopefully future feedback when you played the game..
Michael/Tigerskunk