I have just had a chance to play with this game a little, and I like it very much! But I do have a suggestion for you that would improve the game. The sound effects in Pengo could be done using sound lists instead of CALL SOUND statements. Then they would play smoothly and without the brief hesitations that occur in the game as it is now. The main sound list player can be set to loop the theme music and the second sound list player can play sound effects as needed by the game without interrupting the theme music. You would need to use XB256 to access this feature. Sound lists and sound tables are described in the XB256 manual starting on page 12.
One thing you need to know is that you cannot paste the code for the sound table using "Paste XB". It has to be merged from disk. 1980gamer has used sound lists in his games with good results.